heater wouldn't kick on

Mine was doing the same thing. Turned out the be the gas. Alive. Had to completely remove the Atwood furnace, but an easy part to replace. Cost was about $70.00 and hour of my time.

Changing t-stat setting from auto to on would have NO affect on furnace fan....that setting only controls A/C fan.

DC power from t-stat controls furnace......fan comes on to purge heat exchanger prior to power being allowed to open gas valve and spark electrode.

Generic info...LINK
(RV Furnaces pretty much all work the same)

I always check systems BEFORE a camping trip. But if the trailer was sitting for awhile along with the above, check to see if the tube is clear: ie. no spider webs, etc. If all is clear, then there would be another issue. Be sure your battery is good (12V) or you are connected to shore power and are getting 12V. Be sure you did light your stove 1st so you got the propane flowing. It might take a time or tow to get it going, until the propane gets down there. If still nothing happens, you might need professional assistance or assistance from a fellow RVer who has greater experience.

First thing to check is to light a burner on your stove to get all the air out of the lines from disconnecting and reconnecting the propane lines when you refilled your tanks.
If no propane present, turn off each of the valves on the tanks and turn them back on slowly and recheck the stovetop.

If all is good there check your 12 volt voltage. If the battery is low it will not turn the fan fast enough to trigger the sail switch which then triggers the gas valve on the furnace.

You can scroll down through this troubleshooting site. It lists various conditions .

http://www.ducktec.com/furnace-trouble-shooting.html

Here is a list of pdf's for just about everything related to RV's.
pick what you need.

http://bryantrv.com/docs.html#furnace

I'd suggest to post your problem in the Tech Issues section and be armed with make and model of furnace and thermostat. There is too little information in this post to help you with any repair or service advice.