All Trucks, Campers, Trailers, RV's and Motor Homes - 5th wheel brake light problem

ALL travel trailer lights work fine with my truck headlights off. Everything on the trailer continues to work normally with truck headlights on except when I hit the vehicle brake the left and right tail lights go off and no brake light but the running lights around the trailer, left & right signals and flasher continue to work normally. This happens ONLY on the trailer, the vehicle rear brake and tail lights continue normally with headlights on.

I WAS having with tail/running lights as well and this morning I removed 7 pin connector on the vehicle and checked the wiring and some strands of the #4 black (battery charge) wire had pulled loose and were touching the center #7 yellow (auxillary/backup) & the #3 green (tail/running) pins. I cleaned this up and reattached properly and everything now working except what described above.

The opposing 7 pin connector from the trailer is molded on and is not removable to check.
Any help appreciated.

Check, clean, and re tighten the ground connections on both the truck and RV. If you want to check before clean up, take a piece of wire and temporarily connect it between clean metal on the truck and clean metal on the RV. If the lights suddenly work, do the clean up described.

That got it. I removed the tail light assembly's and the ground wire on both was corroded really bad where it contacted the metal. I cut the wire back to fresh and sanded the corrosion away, reconnected and working great... REAL BRIGHT.
Thanks