In the last 5 years of full timing, I have had *maybe* 20% of my time with the luxury of full hook ups. (I got em now..and am liking it LOL!) I get along grandly, even in the NW woods, with only 150 watts of solar
and a single group 27 battery
because I use old school methods to use less electricity when appropriate, safe, easy and cost effective.
Do not dismiss the hot water bottle as too simple, or as a joke solution. It works. Technology has its place, yes, but I encourage anyone boondocking
to do what Grandma did when it's cheap, easy and reliable first. (Not to mention as heavy)
I too, do not like to run the furnace
at night for power reasons and safety reasons. I got a hot water bottle 4 years ago, and it is one of my "I'll never let this go..get away from it or I will sick my dog on you" kinda things.
I boil water (Yes, BOIL), put it in the bottle, toss the bottle in the bed with an old heating pad cover over it, and the bed is toasty as all get out when it's time to retire. The water is still hot in the morning, even in below freezing temps. The bottle is placed or rests somewhere on my torso, and it heats up the rest of me almost instantaneously. I use good bedding and I have not had a freezing night yet.
To be fair, when I have hook ups, I use a heating pad with the same results.
Could you somehow insulate between your cab roof and the underside of the cabover using a temporary rigid foam when parked? That might help some.