In my winter RVing, C motorhome with unheated basement space, I go through about 3-4 gallons a day (12-16 pounds?) LPG trying to maintain 65F during 20 degree nights. Furnace is off during the day when I'm moving, temps in 40-50 range and sun comfortably heating the house.
Because I'm moving, no skirting. This is about going through the colder parts of Texas to get to someplace a bit warmer, like 40-50 degree nights.
Some RVs do a lot better than others, I know mine is not four season and I would not try to live in it where winters are cold. In the case of mine, and other 2-3 season RVs, it is not just about the amount of fuel used by the furnace. Problem is, furnace may not be capable of using enough LPG, i.e. sized too small for winter heating requiremnts. Of course, you may be able to upgrade the furnace with one sized for a larger RV, but there are limits as to what is available. Larger RVs, particularly four season, use multiple RV furnaces or higher capacity hydronic heating systems.
In Maine, I think I would opt to build a small well insulated cabin, an probably try to heat the space with a good wood stove. My brother gets enough wood to heat his house all winter in Michigan for what I'd pay for a month's worth of propane, buying it in 20 pound bottles.
