Hi Lurker52,
No need for a cheap heat in most RVs just replace the return air grill with a dual window fan. That will keep air flowing through the furnace duct work and is a lot quieter than the furnace fan running (even at 1/2 speed). I added a mechanical thermostat so I don't need to monitor the temperatures.
For heating sources I use 1/2 size oil filled heaters that are essentially silent other than the clicking of the thermostat. I found places for them in the RV that were otherwise wasted space. Total cost under $250.00 for parts, including upgrading all but 3 of the 120 volt outlets in the RV. Time to install is minimal.
And, if need be, my home brew system can be used at the same time as the propane furnace saving dramatically on propane costs.