For ages, I've read posts and watched vlogs featuring content on the variety of ways the Camper-Life community is using to heat their rigs. My rig has the original, perfectly working furnace installed when it was made. My issue with it is that it guzzles propane.
Thankfully, it also has its original, four-burner cooking range. Whether I'm using the burners the oven, or both, it only sips propane. A single, gas-grill sized tank will last for months, using it daily, and leaving the pilot light on.
Below is a vid showing how I use the stove in the rig, safely, as the heating option.