yep, that's the basic layout/flow of the warm (one duct) furnace air on classics...
i have not altered the oem pan/insulation it is simply too much trouble and risk gaining access.
the entire tank pan could be wrapped externally with bubble foil and aluminum tape (that should survive 1 winter season)
leaving the water heater ON gas provides a lot of warmth (and conducted heat) for not much effort.
the external drain valves for the waste tanks can and will freeze.
depending on volume IN the tanks this seems to happen at about 20 degrees F.
of course they do thaw eventually, so my approach is to NOT use the dump valves until reaching a warmer location.
other folks dump a gallon of pink stuff into each EMPTY tank and hope some of it reaches the valves.
the freshwater feed line to the pump may freeze or get slushy.
this happened while towing once, at 10-20 degrees for a daytime HIGH.
again reaching warmer locations solves this issue too.
i do use the furnace when towing, primarily if the daytime highs are 30 or less.
taping the entry door (inside or outside) helps a lot.
since these trailers ALL leak air badly, one can use a LOT of lp gas towing in frigid weather.
2x40 lbs and a 20 lb spare in the truck.
keep furnace set to 40-44 as night time approaches if still on the road,
and once parked crank the furnace heat up to 65 (boondocking) or include space heaters IF connected to juice.
a lot depends on the winter camping goal.
IF simply taking a weekend trip it might be best to winterize the plumbing with pink stuff.
then carry drinking water, skip the shower and use a porta pot or trash bag in the toilet bowl...
IF traveling cross country with the occasional frozen night, use all systems on 'self contained' mode.
full time winter living is an entirely different beast and pipe wrap heaters, skirting or other residential tricks are needed.
there are several old threads on towing in d' polar belt or occasional winter trips.