Just finished (I hope) restoring a 1981 truck camper, so I feel your pain.
I got a new LP alarm that is main TC battery powered, and installed it at floor level. ( There was none before) There was a CO alarm powered by AA batteries mounted at eye-level a previous owner had put in.
I asked at my usual RV shop what level the CO detector should be at and he thought up high, so I asked why they had all these combined LP and CO detectors for sale, and where should they go? He shrugged, and said personally, he would have two different ones so the LP could be at floor level and the CO higher. I have no clue what is the correct answer on that, but that is what we now have. The previous CO up high and the new LP alarm down low.
Had to toss the old three way fridge. It blew GFCI on 120 and would only do pilot on gas--no prime flame. Rusty parts, too much trouble. Gave it a good try, no joy. Yanked it ( 1981 3.6 Dometic) out and got a 3.2, 120v fridge instead. (Have a big inverter and enough battery and solar for that when off grid)
The water heater is old time, gas only, but works so kept that.
The 1981 furnace lights ok, but don't know yet if it maintains the set room temperature. We have a 120v heater if needed, but that will really hurt the battery bank if we have to use it off grid despite the solar.
Anyway, you are in for a lot of work, but it can come out right if you keep at it :)
