Please add your RV make and model, furnace model number for best advice.
Did you check the thermostat?
clean and check the furnace vent, if not protected with a screen, wasp nests.
Sail switch may be stuck and that safety feature will keep it from lighting.
No LP pressure? did you purge the lines before trying?
Some have a fuse in the furnace
If the furnace has power and LP did you check that it is getting a signal from the thermostat?
Plugged or very dirty filter or heat outlet?
There is more but I can't remember them, add your model number and others can help.
2017 SLX 195RB
2014 Jeep Grand Cherokee Overland, gas 3.6 V6
Andersen WDH hitch
Dometic Milenco Aero mirrors