I'm having an issue which I honestly had not anticipated. It might be a little off topic but maybe this will help someone else with their heater project so here goes.
I have a shell model FWC, which came with the upgrade options of propane heater and stove, and the base-level battery. I am not looking at it now but I think the battery was a 90Ah 12V deep cycle. For my first trip I just used an ignition-hot 12V source on the truck's upfitter outputs wired straight to a temporary trolling motor female plug with around 15' of 10ga wire, plus whatever wiring there is on the camper side. When the truck was running I'd see about 13.8 to 14.0V at the camper battery, but it was just not able to push enough current to really charge the battery while I drove. I know this isn't a battery thread, but what it did was give me trouble keeping the original propane heater running. I think what was happening was the fan would slow down and the sail switch would stop working right, and it would trip off. After getting familiar with the propane heater I pulled it out and switched to an espar diesel heater (which I have been running K-1 kerosene in). I really haven't needed the battery for anything except running the heater, so far. I use the lights a little, but they are efficient LEDs and consume almost nothing in the 20-30 minutes per day I've had them on. Without the battery charging fully, I still wasn't getting much runtime out of the heater, to the point that I wasn't sure if I'd be able to run the heater all night for a couple of single-digit nights in a row without really stressing my camoer battery. So, I got a 160W solar panel and a Redarc 25A dual-input charger which does DC-DC conversion to take whatever it gets from both the panel and the truck, and outputs a proper charging profile to charge the camper battery better/faster. I also upgraded my previous temporary 10ga wiring from the truck and replaced with about 9-10' of 6ga straight from the truck battery (fused for safety). The Redarc DC-DC charger has an isolator function which only takes power from the truck once it sees higher voltage from a running alternator, so no danger of depleting my truck battery while parked. I also put in a Victron bluetooth-capable battery monitor (so I can see what's going on with my camper battery even while driving) and a Victron battery protector (to disconnect my camper battery before I deplete it enough to cause damage). This might be a little overkill for such a little system but I already had the Victron gear from another project and it will probably come out if I ever sell the camper. I also removed the Automatic Charging Relay from the camper, which was supposed to disconnect the truck from the camper when the truck isn't running. It wasn't hurting anything by being there, but it was one more thing to fail if there's an issue and it was also redundant since the Redarc now serves that isolator function.
That was a lot, definitely more than I'd intended to do when I decided to simply upgrade my heater. By the way I also dove into making a roof rack to hold the solar panel, which was a whole other adventure. But all good, right? Surely all these quality products will solve all my heating and heating-related charging issues? ...Nope.
I driveway tested it all, even sleeping in the camper all night in the driveway on a -4deg night, and everything worked. But I was parked under a tree in New England in Winter and only got about 90 minutes of good sun per day on the solar panel, not enough to replenish the battery. So I cheated and plugged in a 120V battery charger for a couple hours, which proved to be the fatal flaw in my functional test. I headed out for a 3-day trip with overnight lows in the single digits and dipping below zero one night. What I discovered was that below roughly 30-35 deg, the vented battery compartment (where my Australian DC-DC Redarc converter is mounted) gets so cold that the charger shuts down and won't charge from solar OR alternator. I temporarily addressed this by putting hot coffee in there to raise the daytime temps a few degrees. I less-temporarily solved it by stuffing a 12V heated jacket into the battery compartment on the low setting. If I've got the Espar heater on it's fine, but I'm only in the camper when it's dark and my truck is off, and I don't really want to run the heater while I'm driving around, so that's not much help. I think my final solution will be to use a little 12V patch heater glued directly on the DC-DC converter, probably with some kind of a temperature switch so the heater won't drain my battery and a plug so I can electrically disconnect it when it's not needed.
Anyway, just thought I'd share the trials and tribulations of my heater adventure in case any of those issues might apply to anyone else out there. I'd recommend looking closely at ALL the conditions in which you want your setup to function, and making sure everything will work when and how you want it to, instead of getting there by trial-and-error as much as I'm doing.
And one other potential issue with my setup before someone calls me out on it - my 25A charger is capable of up to 25A bulk charging. My little battery is rated to take 9A continuous charging, 18A surge. Practically speaking my 160W one-panel solar array will never supply that much but the truck could, if the camper battery is low and some other conditions are ideal. I don't think it's likely and I've never seen even as much as 9A into the battery while I've been monitoring it, but I'm taking a calculated risk which I'd be remiss not to mention. It's probably proper to say as a disclaimer, that nobody should do anything which they are not qualified to do, electrical or otherwise. And your mileage may vary.
