Yes, toe/kick space heaters tend to be expensive. Most fan-forced ones are (wall or ceiling). Technically/functionally a good choice.
Permanently mounted heaters should be on their own dedicated circuit. You can run multiple heaters on the same circuit (depending on total wattage). If you want to use the accessible 20 amp wire that is there, you should permanently disconnect the receptacles on them.
You can also install a fan-forced wall heater if you have a suitable location and if it's near your panel, that helps. A ceiling fan heater will also work and I installed one in our last unit. One challenge can be where to install the thermostat. It's best if you can totally conceal the wiring, but not always possible. With some effort and creativity, you may find a feasible route in the ceiling, walls, accessible spaces like behind shower or bathtub, in cabinetry and in the fridge cavity if you pull it out (which I did). You can use low voltage wire (need a LV transformer and relay) to make it easier to install (which I did).
It's really nice to have a permanently mounted electric heater as opposed to a portable plug-in one. Safer and doesn't take up floor space. Won't come close to putting out what your furnace will but it's great for times when you just need a small amount of heat and don't want to burn up propane. Also, can be MUCH quieter... If you install larger wattage heater(s) and you have to plug a 50 amp service into a 30 amp pedestal, you may not be able to run the heater(s).
There's a number of manufacturers of the kick-space heaters (King, Ouellet, Dimplex, Qmark, Broan, Cadet, etc.) I'd buy online if you want the best price. Or - I normally buy electrical products at a wholesaler for about 1/2 of retail price. Many of them will sell to non trades persons. On some products you'll also get better quality compared to retail places. Dimplex says their heater is "RV compatible". I doubt there's anything special about them for RVs.
One consideration with any fan forced heater is noise. Some are better than others. If you buy the cheapest one, you may be disappointed. There's one brand call the "Quiet One" which claims to be quiet.
Quiet One heatersSome heaters have a delayed start and stop and need a 3-wire cable so you should be aware of that too.
Wiring of heater with delay featureIf you're handy, I think it's well worth the effort.