The furnace will not have its own fuse. It is likely on the same circuit as other 12v
items. There are 12v
fuses and circuit breakers in your fuse box.
Test to make sure all 12v
items work in your trailer; lights, fans, water pump, etc. If so, then it's not likely a fuse. Usually there are 4 wires to your furnace. Two are power, and two are thermostat. The power wires are larger in diameter (gage). Your wiring diagram may tell you what color the ground wires are. In my 66, they are white. Check your ground wire from the furnace and insure it is in fact a good ground. It should test at 1 ohm or less. Then check for 12v delivered to the furnace. That ought to be 12.5 volts or higher.
I might add the thermostats in these old Airstreams can cause the furnace not to run. Make sure your thermostat does function. You can "jumper" the two thermostat wires together and your furnace is commanded to run. If it doesn't, then you are back to the 12v power to it.
It is likely to take some dissassembly of cabinets, etc, to get reasonable access to your furnace. Please turn off your propane while working on the furnace. Once you get your fan motor to run, then you can worry about the pilot light and other furnace functions.