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Seems like a loose or intermittant connection in the furnace .....especially if banging on the side of the furnace access door outside starts normal operation. Check ground and 12 volt connections are good and tight at the furnace end. Also check if closing the outside furnace door causes any intermittant short (pinched wire) somewhere in the wiring. Does the air conditioner come on normally when the coleman mac is set for cooling and you temp set call for cooling? If both heat and cool are intermittant then the 12vdc connection, source or thermostat are in question.
I converted my coleman mac to digital many years ago and did a drawing related to that conversion which I will try to attach for reference (left side is the analog coleman mac I used to have). If our coleman macs are the same, I think the red wire is the +12VDC coach battery, the Blue is Ground and the White should be the switched +12v feed to the furnace when heat is called for (coleman mac temp set above current room temp; heat switch on if applicable).
When thermo calls for heat it should short red and white leads to provide +12v to the furnace on white lead (but not sure if that is how your coach operates or is wired). Strange that you say when “not working” the red to blue is only a couple volts........you mean thermo is calling for heat but nothing is happening at the furnace and you only see a couple volts measuring red to blue? If yes, sounds like a heavily loaded circuit dragging voltage down but you would think the thermostat fuse would blow if true (depending on current flow of course). The load is the furnace control board, fan motor etc. (assuming a faulty thermostat is not causing the voltage dip somehow). You could try bypassing the thermo completely with appropriate in line fusing between R and W, but banging on the outside furnace cover seems to imply outside furnace wiring issues.
Sorry I can’t help more.
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