Finally had some time to get my campers electrical system tested. I wanted to do that while the roof plywood was off. So any repairs would be much easier if needed.
Glad I did the test. I did find that the aircraft type reading lights I installed last minute were wired into the wrong circuit. I had made a mistake and tapped into the wire that fed an outside porch light. So you would have to turn on the porchlight outside, to use the reading lights inside. Not the best setup. It was an easy fix with everything open. That area would be impossible to reach with the roof panel on.
The rest of the 12v system worked great. Lights, USB charge stations, vent fan, 110v power inverter. All worked as they should thankfully.
In other bad news I had a puzzling fail with all the road lights. Plugged into the tow vehicle, and only thing that lit were the two pass side tail lights. That's it. Then when you turned on the right turn signal all the side marker lights flashed. I had nothing from the left two taillamps at all.

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So after much test light probing it turns out I appear to have a bad 7 pin connector harness. First off is after probing with the test light, the harness wire colors are wrong. When I moved the wrong colored wires from the harness side of terminal block to match with the correct color on the camper, everything lit up just fine. Al the running lights and the right signal and brake lights worked correct too. Still dead on the left side tail lights. None of the wires from the harness, right or wrong color, have power for the left signal lights.
Just before taking a break for lunch, I unplugged the tow vehicle and pulled up to another trailer I have that uses the 7 pin plug for wires. Plugged into it and everything worked just fine. Including the left signal/brake light. So I have eiminated the tow vehicle as the problem.
Going to get out there today and run some continuity tests on the connector and harness today. I'm 98% certain it's the problem. Sad thing is the harness has a molded on plug on the tow vehicle end. So I can't see which terminals have what color wires connected to them. I think this brand new harness is bad. Today will show for sure if I'm right.