Furnace will not light

Thanks for the help from everyone. Thanks to the replies, I found the problem, bad thermostat.

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.

Please stay on topic.

What make? Suberban? Does the electronic ignition engage, click click click or not. If no click make sure the igniter is not obstructed or contact very dirty. If so clear and clean. Otherwise you may have a bad ignition circuit board, Been there done that. $100 bucks to replace. Unless you can find used, basically any suburban board will do, furnace, water heater don't matter if that is your brand which most were back then.

Post deleted for health reasons. (As moderated, it was nauseatingly twee.)

light the stove let lit for about two minutes, that,ll bleed air out of the line, retry furnace , make sure you open the tank valve. and be a little patient blower will blow then it should light. light.

What exactly does the furnace do or not do? Does the fan blow but the flame not light, or does it fail to do anything at all?

I assume the answers are probably "yes," but...do the other 12V systems work properly? and likewise the other propane appliances?

Welcome to RV.net!
Check the thermostat for on/off switch
Next remove cover from the thermostat, locate the two wires going to the furnace, connect them together, furnace should start and produce heat shortly.
Didn't start? Open access for the furnace. Look for a circuit breaker mounted to the fan housing. Should have reset button.
Check for incoming power.
What make and model of furnace?