I didn't research SRW so I have no idea what the costs would be there. I don't know any of the engineering implications there either, but there have been some detailed posts on that issue.
I would recommend an aux fuel tank.
yes
An aux fresh water tank really helps a lot too. The campers have pretty limited capacity.
yes
For aux. batteries you'll probably spend more than that for one battery unless you are adding very little amp hour capacity.
yes
I recommend AGM batteries and three stage charger/controllers.
yes
I would also say: Don't skimp on that system. If possible, use top quality remarine components and marine wiring.
yes
Make certain, FOR SURE, your controller / charger has a profile to match your battery before you pull the trigger, especially if you are looking at Optimas. I was able to program all of our controller / chargers to optimize them for our battery's characteristics. If you mismatch those components you will signficantly shorten battery life and compromise the output of the system.
yes, thoe i have heard mixed reports on optimas
For sizing, remember that you only discharge to 50% of capacity, and you'll never get 100% effeciency. For instance, if you have a 100 amp hour battery, you only want to use 50 amp hours out of it before you recharge. And you'll never get 100% effeciency out of any component of the system, so you will probably net 45% best case.
true
A battery combiner system that uses the stock alternator is probably your lowest cost option for recharging.
yes
If you are planning to use this off the grid, and I assume you are or you would have bought a regular RV, then solar panels make a lot of sense. Again, get a 3 stage controller / charger that you can match to your batteries.
yes
And BTW, matching to the batteries goes way beyond "does the controller / charger have an AGM setting for my AGM batteries Y/N". You need the ability to exactly match the voltages, amperages, hold times, etc. between your controller / charger and your batteries' requirements. Anything less and you are paying for a lot of expensive amp hours and are not getting all of them or you are systematically destroying a very expensive deep cycle battery.
true and yes
And here you thought this would be simple, right?
it has to be simple, for me.
I remember when I started down this path. "Hey, we're putting a camper on a truck. How hard can it be?"