Big thread on my snowmobile trailer -> snowmobile camper conversion here:
Big things were insulation, rewiring (12v and adding 120v), and the furnace install.
Cleaned/undercoated the steel:
Pulled out all the old wiring, installed new wiring, and insulated the walls:
Box on the front for the regulator/hookup
Insulated the ceiling
25k BTU forced-air furnace
Battery... removable so I can keep it on a tender in the garage instead of leaving it in the trailer and freezing. LPG/CO detector too.
It does well.