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The first thing I would do is confirm the furnace manufacture's name, model #, BTU #, serial #, etc., off the furnace label...., you may need a flashlight and mirror depending on the location of the label. Visit the manufacture's web site (or contact them directly) and get a copy of the furnace operation/maintenance/installation manuals......, they should include a detailed parts listing.
Detailed product info will be extremely helpful when diving into an RV furnace and help with a YouTube DIY search.
Common issues with furnaces:
* Not receiving LP
* LP gas valve at furnace (defective)
* Exhaust port clogged (mice nest, cob webs, etc.)
* Restricted return air duct (if equipped with one)
* Wall Thermostat (defective)
* Sail switch (defective, or debris like pet hair, etc., restricting paddle movement)
* High limit switch (defective)
* Circuit board (defective)
Some RV furnaces have a small relay located on the A/C depending on the type of OEM wall thermostat and brand furnace & A/C system combination installed. You can hear a delayed 'click' (normal) if standing close to the interior ceiling A/C vent cover when turning the furnace on at wall thermostat. This type of system is more prevalent in newer RV furnaces.
If the furnace has an outside wall access panel, start your inspection there before removing the furnace, many components can be serviced from this open area.
Moderately tapping the furnace while someone turns the furnace on from the wall thermostat may free up debris around the 'sail switch'. Also, many RV furnaces after 3 failed attempts at starting will require a 'reset' at the furnace.
Hope this helps......