Well at least you or family have not frozen....yet. The main thing is to make sure your pipes don’t freeze in the coach.....people can huddle around a space heater or put on more clothes to stay warm. You may want to call a mobile tech as that may be less costly than a major freeze and water leak in your coach..... or a divorce. LOL.
In the mean time, if the weather permits (no rain/low winds), I would be tempted to leave the furnace door off in the day time while you standby for it to fail again.......and then carefully move select “start circuit” wires (like +12v and ground; on off reset switch or thermostat or control board; motor) in the furnace area to determine if/when the fan starts up (during a constant thermostat call for heat of course). If no intermittant wiring trouble is found then try your tapping technique around furnace devices to home in on the fault location.
You said previously that you measured a few volts only (between red to blue on the inside thermostat during the failure time – which is essentially +12V and ground per your schematic). For voltage to drop to a couple volts (implies high current loading or high resistance intermittant connections) is not normal especially with no apparent fuse blow or smoking components. Maybe just a bad meter reading ......or flakey high resistance +12v or ground wire connection or wire conductor itself causing intermittant high resistance conditions.
One other option which requires the +12 feed turned off to the furnace (fuse pull best at source) is to use the ohmmeter setting on your multimeter and place it across individual wires that should provide short ccts (low resistance on the meter) and then wiggle each wire being measured to see if the meter reading changes). This could indicate a bad wire or broken conductor inside the insulation. It is best to have small alligator clips on the meter leads so you can clip on each wire end and wiggle wires. Ohmmeter readings must be taken on dead circuits – reading a live circuit will potentially damage your meter. Remember to reset meter to read volts again when you are done with any ohmmeter tests.
That’s about all I can suggest.
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