The random nature suggests several possibilities. If the cold air scenario lasts for about 25 seconds and then the blower shuts down, it could be the sail switch. If you have the air return or any of the ducts blocked, that could be causing a sail switch random failure.
If the cold air, when you get it, blows continuously, it could be the high limit switch intermittently failing.
You might have a loose wire. There are typically 4 wires from the trailer wiring connected to the furnace wiring. A bad connection on one of those could give you intermittent results.
It also could be the control board. If your furnace is mounted horizontally, it's on the left side. The 4 screws holding the exterior cover have to be removed to get a look. There are probably 2 long screws that mount the furnace assembly to the frame that's alongside the furnace.
To get at the wiring or control board, you probably will have to disconnect the propane supply and then gently pull the unit out a few inches. Handle everything gently.
I'd inspect the wire nut connections first before going to the control board.
The sail switch can be accessed by removing the large half round cover over the blower assembly.
The high limit switch is on the other end and I think you might need to completely remove the furnace to get to it.