The rule of thumb for propane furnace operations if you are running off the battery (No hookups) is ONE NIGHT ONE BATTERY. Since you have two propane furnace furnaces perhaps your battery is too low to run the furnace fan at the proper speed...
The furnace fans all run from 12VDC so thay can be used when you do not have electric hookups...
If the furnace fan runs to slow then the sail switch doesn't work properly and this shuts off the propane flow to the furnace... ???
Something to check out at any rate... Battery may be run down below the 12.0VDC reading. A typical fan blower will draw 5-6AMPS DC current and if you have two fans running that is 10-12AMPS DC being drawn from the battery all night long...
Nose your truck up to the trailer battery and use jumper cable and see if that makes it work...
Roy ken