Yes, when the propulsion engine or genset are running anyway, I definitely want to take advantage of heat produced for free.
But when stationary free camping for maybe weeks at a time, will directly burn vehicle fuel or propane, that will be by far the main source of heating BTUs.
But also load dumping "free" excess solar electric to a 12V DC heating element once the lead bank is past say 85% no longer accepting many amps. In fact same with production from the DC-only generator when there is excess there.
I have some off-grid forested land I may put a container-based tiny-house on, so that might add a wood-furnace heat source when parked there, but I would not bring that bit along when travelling.
The Australian Kimberley Karavans and Kruisers, as well as other campers down there, use the Webasto ThermoTop for hot water and space heating. I have one in my Kimberley Karavan and it works quite well. Space requirement is quite small and it is very thrifty with diesel fuel - two gallons will easily go a month with everyday use. It does require an external outlet for the exhaust.