Furnace

The blower starting triggers all of the other functions of the furnace. Most furnace control boards have an LED on it that will flash blink codes to help you troubleshoot. If it is not lit up, there's no power to the furnace or the control board isn't working.

I had my furnace fill up with water due to an improperly mounted exhaust port, and the blower still worked, it just wouldn't stay lit. Definitely make sure power is going where it is supposed to go. I hate to say this, but if you find power at the terminal block and you can't get at the control board, you are going to have to pull the furnace out to troubleshoot it. Fortunately it is easier than it looks once you find all the mounting screws.

If you've done all that has been suggested then it's time to get out the volt meter and start tracing some wires and voltages from the thermostat to the furnace.

I will give all of these a try. I understand that if the blower won't turn the furnace won't ignite, hence no heat.

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

You really need to measure voltage with a meter at the furnace board. If the time delay relay has failed or doesn't have power, the blower will not turn. Blower no turnee, furnace no workee.

I did flip the mechanics switch off and they're was no difference. My thermostat is not the digital sort. No buttons, just switches.

Vancil wrote:
I have swapped out all of the fuses that I can find for the unit.

If you don't get the blower to operate you'll never get the furnace to heat.

Vancil wrote:
Disconnecting power and resetting thermostat caused no change.

That won't reset most modern thermostats as they are powered down every time you disconnect the battery. They need a specific set of keystrokes to be reset. Here's the reset for a particular type of Dometic thermostat - Reset

Also, you should try flipping the "mechanic" switch to off and see what happens.

I have swapped out all of the fuses that I can find for the unit.