Atwood or Suburban furnace?
I just today had my furnace fixed by mobile tech--same symptoms, blower would come on, run for 15-20 seconds and shut down. Burner would not ignite. It was the high limit switch. If you have the Suburban
(mine is the SF 42, pretty common) then the furnace has to be pulled completely out of its chassis--no need to pull the chassis with the ducts on it--to access the switch at the right rear of the burner flue.
Mine is the direct spark ignition--no pilot. Part was $25.00. After seeing it done, will be able to do it myself if it happens again. Without seeing the process, I would have been disconnecting the ducts and trying to pull the furnace AND chassis out together. It is an awkward deal as the assembly is a tight fit and can hang up on the duct blanks that aren't in use.
Hope this helps a little.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired