Your post makes little sense.. I will make an assumption and try to post
You said it had a propane heater inside, Your daughter ask why, and the seller said one time he turned IT (Assumption you mean the furnace) on it was smoking.
Three things can cause an RV furnace to smoke.
1 a short in the blower motor can indeed let smoke out. Very rare More likely to blow fuse or trip breaker.
2: Insect/creature nesting in the path the flame/exhaust takes causing all sorts of problems. In fact I think this is likely the prime suspect.. Routing cleaning may include a nice high pressure/velocity air blast to blow that "stuff" to .. elsewhere.. may help.
3: Improperly adjusted air gate or dirty gas jet
Oh there is a 2.2 as well,, Dust accumulation burning off but this passes swiftly on a Furnace.
The reason for the assumption: The way you worte the post "IT" can only refer to the propane heater or to the trailer since those are the only two ITs in the post.. You failed to mention the factory installed furnace/heater.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times