We have a Keystone 245RL 5th wheel, advertised as "Four Season" ... in Colorado it's really a 3 season. I use it for hunting, but if I'm into the later seasons and up high (sub-zero nights and snow) I winterize and use jugs of water.
The underbelly heating (on mine at least) is 1 little heater duct that sort of points towards the waste tanks, and the insulation is a very thin sheet of batting (I intended to upgrade that the last time I dropped the belly but ran out of time - still on the to-do list.
I don't run the furnace while I'm out during the day, only during the evenings. I do augment with a generator (Honda 2000 portable) while I'm inside, to run an electric heater (which is counter productive for the belly heat, because when the electric space heater is running, the furnace is not putting as much heat into the belly) and to recharge the batteries - I have 3 12v batteries - solar also on my to-do list.
I'm honestly shopping for a smaller slide in camper for hunting (keeping the 5er for family outings), as I want to tow ATVs and/or snowmobiles with us and my wife is paranoid about double towing.
