Let me see if I understnad right.. The "Left over wire" goes into a harness and off somewhere into the motor home right?
If so.. Start by measureing voltage with respect to ground. If you have some (Do this both with engine running and off) it may well be a positive wire
It may also be the wire to the AUX-START relay.. The steps, or the propane detector
All these are positive
Odds are very small it's a ground.. but not impossible.
One tool I think every RV tool box should have is a test lamp IN fact you need two but the other one is a bit tricker to build being a 120 volt job.
For the 12 volt job you have two choices, You can buy one ready made, but I've not had much luck with those
Or you can home brew.. I started with a LIcense plate lamp socket, this blade type socket has two leads (Hot and ground) and I put a common alligator clip on one.. the other I extended out a few feet and then put a clip on, Soldered the joint and taped it up (Well heat shrink tubing.
Put the matching 'Dash size" lamp in it and I've got a test lamp
This draws enough current that I can test stuff "under load" but not so much as to blow things up
Hook this lamp first positive to suspect wire.. No light means it's not a ground. Hook to positive
NOTE: DIM light also means not a ground
BRIGHT light.. Might be a ground (Still not sure)
In a few cases I had to build a bigger test light.
Home is where I park it!