The guy you want to ask is Randy Bishop: Fiberglass RV - View Profile: Randy Bishop
He has repaired several of these furnaces.
But here is what I know. The pilot should go out when the furnace
is in the off position. This is a must. If it does not, then disconnect the furnace
and cap off the propane
supply. The problem is likely the gas valve. Randy says he has never had to replace one. They just need to be cleaned mostly. There are replacement valves though. The problem is that they are not exact replacements. The ports are in different places, so some tube bending is required. Trillium
in Calgary put one in the furnace
on my first 4500. It was not cheep. Here are some links to the replacement valve:
Duo-Therm Furnace Gas Valve, Replacement 710-201MC
If you don't have a tube bender, you should probably find a gas fitter, or RV technician that can install it for you.