I think this comes down to do you have AC or not. With AC I have a ceramic heater if not I have an Espar/Eberspacher diesel heater, not cheap but low power draw and safe.
If I did not have this and wanted to build something my self inexpensively I would use a closed loop externally fired boiler and a auto heater core. That external boiler could be an RV gas water heater or home made wood/solid fuel fired. The home made version could also use a heater core. The one fly in the ointment would be feeding a small fire and regulating heat output and yes dampers can be used to lengthen burn time and reduce output. Or you could use a propane burner. Why a liquid heat exchange medium, there is no danger of smoke/CO inside the teardrop and hookup can use quick disconnect fittings.