[kd2iat]

Here's one possibility...

The RV manufacturer purchased a stock of cable with x number of conductors, but only ended up needing x-1 for your rig. Therefore the dangling wire. I found that to be true on two different rigs over the years. If you can locate the other end of that cable, you may well find the same dangling orange wire. Proper wiring practice suggests that unused wires should be taped back or cut away in some fashion, but a lot of RV manufacturers tend to skimp on these practices. Just pull any 110vac wall socket and examine the wiring. It ain't up to what we expect in residential or commercial wiring.

I'd pull the thermostat off the wall and see if the cable feeding it has that extra wire. It's also possible that it's in a bundle going to the furnace.

As for your furnace not be properly controlled by the thermostat, something's not connected right would be my guess. You'd probably need to have the install manuals for the thermostat, A/C, and furnace laid out side-by-side and trace out all the wiring. It's tedious and mind-numbing, but necessary. I've done several conversions of the thermostat from the standard Dometic Duo-Therms to a Hunter or other unit. Each time, I ended up tracing wires and wire colors. Seems that RV manufacturers don't always purchase multi-conductor cable that has the same wire colors as needed by the various appliances. They use what they have in stock and that can create issues for owners down the road.