We're camping on this cold weekend and can't get the furnace to start. 2 weeks ago we had the same problem but it finally started after turning the thermostat switch on and off repeatedly. That's not working this time. Any hints or suggestions?

Had the same problem years ago
The pressure regulator at the propane tanks it has a rubber diaphaum that goes bad
Mine was 5 years old and wasn't sending the correct pressure to the furnace causing it not to light
If it is older than 5 years old I would replace it so you wouldn't have further problems and also check the pressure with a gas pressure gauge it should be 11 inches of w.c.
The gas pressure gauge gets hooked up to the output port of the gas valve on the furnace and the adjustment pressure screw is located on the pressure regulator at the propane tanks
When you buy a new pressure regulator , get an automatic change over style for dual propane tanks with a gas pressure adjustment screw to be able to set pressure at the furnace

I figured out the problem late last night. Bad contact in the thermostat switch.

Just go ahead and change your regulator and the propane hoses out. Cheap insurance that you won't be left out in the cold.

Maybe I'm an idiot, but I'm having trouble even finding access to the heater controls on our 20 Funmaker from 1987. It's under the seat, below the thermostat, but I can't seem to see access after that.