You mentioned heating with propane... Will this be with a radiant or catalytic propane heater by chance? (I don't see a furnace fan listed in your list of loads).
Also, 24 hours for a 3-amp roof fan? That's a big fan to leave going like that all the time... (and noisy too)
Finally 60W of LED lights is about the same as 500-600 watts of incandescent... Is that correct? That seems like a lot of light to have in a van (10-20W, or about 800-1500 lumens) is usually the max most people use).
Something to keep in mind about the current draw of a fridge (as well as that of a furnace fan, if that was inadvertently left out) is that it can be very highly dependent on ambient temperature, so there really isn't any set figure that it may use on average. My experience is a fridge in the 40-60 quart size class will use about 450-650 watt-hours when temps hover around 75-85°F, but can be as high as 1 kWh over a day when temps hit 100F, or as little as 50-100Wh if temps remain 60F & below. Something to keep in mind when calculating your loads if you frequent particularly hot or cold areas.
Given what you have listed (and a more typical figure of maybe 3-5 hours/day for that fan and 15W of LEDs), 300W of solar should be plenty sufficient for your setup (you might even be able to squeak by with 200W).
Answering some of your Q's:
- Does MPPT make enough of a difference vs. PWM to be worth upgrading, or should either of these setups be good enough as they are?
- The big sacrifice between the package vs. self sourced options looks like it's the inverter and controller. Are the better components worth the extra $600?
- I know almost nothing about choosing batteries. Do I need to worry about venting? Are there particular brands that are good bang for buck? Do I really need 3 batteries (300 ah)?
- What do the amp ratings for controllers mean? Is it a max capacity and should be fused, or would a 20A controller simply mean I'm not maximizing potential with a 300W 12V system?
- What kind of wiring components do I need to get it all going? Fuse between solar panels & controller, controller and battery, battery and fuse block? Lots of fuses...
MPPT has become considerably less $$$ over the last few years, so at this point in the game I would say it's almost always well worth it over PWM. MPPT significantly improve solar harvest very early & late in the day, as well as when it's cloudy or overcast.
IMO, putting your own system together (vs buying a package kit) definitely can get you a much better mix of components for cheaper. The caveat though is you need to be familiar with what to buy, and to make sure everything is matched well for each other (that the controller can handle the total wattage and/or current of your panels, for example). Another benefit afterward though is that you'll also be much more familiar with the system, which should make it easier to troubleshoot any issues you might have later down the road.
Batteries have become somewhat a topic of contention... My opinion is that ANY decent battery should serve well for years if it is properly cared for (even the Kirkland/Interstate deep-cycle units Costco sells for $80). What causes premature failure of most batteries regardless of their cost, is improper charging (letting it sit at less than 100% charge for long periods of time). Solar generally does an excellent job of keeping batteries charged, so long as the solar system is sized adequately.
On the subject of battery venting... Contrary to some comments I've seen, you absolutely do NOT want to put the battery inside of a sealed box or enclosure, as this can potentially lead to slow buildup of gasses inside the box and possible explosion. Best is to simply have the battery in the open where any gasses created by the battery can simply dissipate into the air (inside of a cabinet or wood or metal box is fine as long as it's not completely sealed to where air can't flow through it at all). The amount of gas produced by any "maintenance-free" type battery is exceptionally small-to-none, so even venting into the passenger area is fine. No odors or dangerous gas will be present except if there is a major charge system malfunction, such as if your alternator's volt-regulator were to fail and force-feed the full output into your fully-charged batteries (AGM batteries can gas in this situation too, so they are to be treated exactly the same as flooded or "wet-cell" units).
Amp (and watt) ratings on solar controllers are to help you size it to your panels properly (or vice-versa). PWM controllers are typically rated in amps (total amps of your panels in parallel), whereas MPPT controllers will usually be rated in watts (but may carry a max charge amperage output rating, which is often the same as a switched load output it may also have). To use an under-rated controller typically means you will not get the full harvesting potential of your panels (most controllers have over-current protection built in, so damage to the controller from too much panel isn't likely to be immediate, but it could still shorten it's life span).
Typically you do not need to put a fuse from the solar panel to the solar controller (the current can never be more than what the panels are capable of producing on their own), but a fuse from the controller to the battery is mandatory.
Many here will wire up a fuse block off their battery to power all of their accessories from a central location, which also makes tidying up the wiring a lot easier... If you do this, theoretically you will want to put a main fuse from the battery to the fuse block that is a total of what all the branch circuits will be, though it probably doesn't need to be over 80-100A unless you have a special need for it (a high-power inverter for example), in which case it's often best to just tie such high-current loads straight to the battery post with it's own fuse instead. IMO the solar controller should go straight to the battery post with it's own fuse, not to a fuse sub-panel.
Hopefully that helps you to get started.
Solar systems aren't too real complex once you start to get to know what each component does.
Edit:
You changed your post while I was making my post...
Grounding to the frame is fine AS LONG AS your vehicle is prepared for it (in other words, you've beefed up the factory wiring that bonds the frame to the battery and the vehicle body, as these wires are usually only #12 or #10 AWG which could be damaged if you were to draw a heavy load off the batteries).
If you don't have a good understanding what's involved to safely use frame grounding, then the easy alternative is to simply use a heavy cable to ground your aux battery bank (-) to the negative of the vehicle (engine) battery, which avoids such issues with the factory frame wiring harnesses.
ALWAYS put a fuse on your loads (this includes an inverter). The fuse protects your wiring (and your vehicle), not the inverter itself. If the inverter has extra circuits to protect it's own self, that's great, but that won't protect the wire going to the inverter if something were to chafe the insulation and ground it out.
If you meant fuse the AC side, if you will be plugging directly into the inverter itself, then no, you should not need anything additional on the AC side.
Inverters are designed to be left wired straight to the battery. When switched OFF (as opposed to "standby" or "sleep", they should draw zero amps (maybe a few microamps at most). So use of the inverter's own on/off switch should be sufficient to prevent the batteries being drained.
Look on the inverter for it's power consumption... If it's listed in "watts", then divide by 12 to get amps... That (or a little above) is what size fuse you will need for it.
You can put switches between the solar panels to controller and controller to battery if you want, though I'd probably just leave the system to keep your batteries topped up while the vehicle is parked in storage. This should not hurt or shorten the life of your batteries at all. Note that when powering up the system with switches, always switch the controller on before switching the panels on, otherwise you might get an error.
And as for having too much system... Too large of a system (not using it to it's full capacity) won't be detrimental to the system itself. But why have all that extra weight weighing you down (and the emptier wallet that goes with it) if you aren't using it?
Anyway, there's some more for you to go on.
Hope that helps
