When you mention *** Do not connect the Blue Wire to O/B *** what is O/B?
Sorry for my ignorance. In your link I think you say to connect the blue wire from the RV to the C terminal on the Tstat.
Wiring Color | Function | Thermostat Label
Green – Fan High –> G
Gray – Fan Low –> Not Connected
Yellow – AC Compressor –> Y1
White – Furnace –> W1
Red – 12 VDC + –> RC
Blue – 12 VCD – –> C
Warning!!!
*** Do not connect the Blue Wire to O/B *** It will blow fuses if you do.
Your devices all share a common ground on the trailer and do not need this to be connected. The thermostat is setup for AC power and does not handle the DC ground properly.