I think you'll find that the majority of vintage RV's were one season ( summer) use units, poor insulation in walls & ceiling , none in the floor , exposed tanks, and single pane windows. Trying to adapt them for winter use , could be an expensive undertaking.
Even modern RV's , sold as "winter ready" can be problematic in cold weather. Poor heat distribution and water lines prone to freezing.
If you're determined to proceed, look for a no slide unit, that suits your needs , as the slide floor and gaskets are the areas of biggest heat loss.
Good Luck in your search.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.