[jsb5717]

The trailer is old enough that it could be a few different things.

1. Faulty thermostat
2. Faulty relay in the AC unit that sends the signal to the furnace
3. Faulty furnace main control board

There should be 2 blue wires that come from the AC relay to the furnace. If you can locate and jump those 2 wires you would effectively bypass both the thermostat and the relay. If the furnace spins up then you can assume that the trouble is one of the first 2 issues. Otherwise I would bank on a bad control board. With exterior access it should be fairly easy to replace.