I bought the trailer used. So I don't know if it has electric heating pads on the tanks (assuming that was an option). It does have dual pane windows which was an option, but that doesn't mean the original buyer also opted for heated tanks. I don't recall having any mysterious switches. Are there are any other telltale signs (other than dropping the underbelly) to look for?
So, my main takeaway from the responses so far:
Use the furnace for heat as it will heat the underbelly. I always leave the WH on anyway.
When hooked up, the only issue would be propane. I have 2 x 40lb tanks (an extra 20lb over most standard RV tanks). Propane would be mostly dedicated to the furnace and WH. Cooking is mostly outside and off a separate, smaller tank.
When dry camping, the fridge would consume some additional propane. Here is where electric would likely be an issue. I currently have a dual 12V system with 135 watts solar. That may not be enough to run a furnace all night. However, wouldn't I only have to set it high enough to keep things from freezing (e.g. 40, 50 deg?).
I'd hate to run a generator at night, but maybe it could run off the batteries and I'd only have to run the genny to augment the solar if I don't fully recover the batteries the next day.
FWIW, I would only be camping for a week at a time. And I'm not talking real cold here. I'm thinking 40s/50s daytime and maybe into the low 20s at night.
2020 Chevy Silverado 3500HD Duramax 4x4 Crew Cab Standard box
2011 Arctic Fox 27.5L