A 3 y.o. sail switch could fail. Mine did.
We bought a new-to-me TT last spring and its furnace had similar symptom as yours except only intermittently. After some brain-storming with a knowledgeable forum member, we pointed our finger on the sail switch. Since I inherited an extended warranty from the previous owner, I took it to a RV shop. The 1st thing the RV Tech did was to test the propane pressure. The reading was 8" W.C. instead of the normal 11" W.C. A new propane regulator was installed and it worked for a few days but only when the furnace felt like it. On the 2nd pit stop, the RV Tech replaced the sail switch which had a short internally.
So you can see there are the two prime suspects that could cause your problem.
Another possibility is a failed main control board. My last TT has a Atwood Furnace. Its circuit board failed just barely out of warranty. The symptom was similar. The fan would start when the thermostat calls for heat. But the spark module wouldn't produce a spark, after 3 tries, the furnace would shut down.
You can rule this out by listening for some clicking sound during the starting cycle. If there's clicking sound, chances are the spark module is working.
