Could you post your RV and the brand and model furnace? Odds are it is a fuse in the furnace or a bad wire connection.
Double check with your trusty multi-meter that the furnace is getting power.
Several you tubes on furnace problems available on the internet.
2017 SLX 195RB
2021 Jeep Grand Cherokee Summit L 5.7L V8
Andersen WDH hitch, Renogy 100 AH Lithium &
200 Watts solar panels from Renogy
Prev. '14 Jeep Grand Cherokee Overland, gas 3.6 V6