Constant 12VDC is supplied to furnace via Fused circuit from DC Dist Panel
That 12VDC goes to the Power connection on Circuit Board (in furnace)
12VDC would then go to FaN (Blower connection on circuit board) WHEN DC from thermostat goes back thru Limit Switch AND Sail Switch to circuit board (triggers Blower section)\
Fan clicking On/Off......suggests BAD thermostat AND Stuck Closed Sail Switch
OR....bad furnace circuit board
IF you have an external access 'Panel' vs just intake/exhaust ports thru side of RV you can access Furnace components.....check for loose wires/connectors or bad wiring
If no external 'panel'...then furnace has to be removed from case inside RV after removing intake/exhaust ports from side of RV