W, mentioning the 12v power test jump started my memory . :)
vancil, Years ago , when i had this problem, the first time i had to tap the furnace while i tried to start it up. I had a flat spot in the motor, which i was told is a common occurrence. I used a hammer to moderately bang on it to free up the motor, while it was on attempting to ignite. After tapping 5 or 6x, it started.
Another time, same rv , same furnace, i turned the unit on and took the rv for a drive, bumpy roads jarred the motor and it fired up.
2nd time it happened ... A few years later on another rv, i had to replace the circuit board.
Being you tried many things, there are 3 modes that that operate things in an rv.
IM not sure if the following would be waste of time or not, but its something i did to try to narrow down the my furnace issue by narrowing down its power source. I tried to operate my furnace using the 3 methods separately. I would find it interesting, if the furnace worked in only one of those modes and not the other.
Standing alone , when you are NOT hooked up to any power just running off the battery.
Plugged into shore power
plugged into the tow vehicle (12v source) and not into shore power