Well.. Try this. DO NOT HOOK THE GROUND WIRE up at first. (Nor mount the "Trailer" end connector on the towed)
Hook it up, Either ground via the hitch or use a standard Jumper cable, frame or negative battery post on towed to frame on RV for ground, and then test the lights and brakes.
if everything works as it should.. Hook up the ground wire.
NOTE: If you have a battery wire, Do not hook it up either. Use a volt meter to measure the voltage on that lead with the Motor HOme engine running.
Normal alternator voltage on the line, go ahead and hook it up, ZERO, check what you THINK is the ground lead.
That is basically how I wired every trailer I ever had, Hook it up and see if it works.
Actually being a high tek kind of guy, I metered each lead, then applied power to the proper trailer lead, verified I had the right wire, and then hooked it up got it right every time that way.
Home is where I park it!