Using propane heater exhaust as a outside heat source?

The simple reason RV furnaces are not more efficient is space- you need a large heat exchanger to extract more heat from the exhaust- if you didn't mind sacrificing an entire closet for the furnace, you could get that 90%+.

Other than the obvious CO issue, the combustion air intake and exhaust are adjacent to each other so that any wind will affect intake and exhaust equally. If you move one or the other, you make it much easier for high wind to affect the flame, with bad results.