Furnace will not light

If you are handy, and can get to the circuit board, pull it out of connector and clean the contacts, re-insert. Also, check 12 volt wiring connections, may be loose or corroded. If you don't hear a sizzle sound when starting furnace, the ignition circuit may not be working. Should also hear a mild clunk when gas valve opens. There may be an air bubble(s) in the propane lines that prevent gas flow and ignition. It sometimes helps to start stove burners or water heater to get gas flowing to furnace. The "sail switch" or gas valve may be stuck. Burner gas orifice may be plugged. Furnaces have many safety features that prevent starting if there is anything wrong. There are You Tube videos on furnaces that may help you DIY. Don't risk fire, etc. Get a qualified RV service to fix the furnace if in doubt.
May cost $200+ if they have to remove furnace for diagnosis and repair.