When this problem initially occurred I took the truck to Chevy and they verified that the problem was in the under hood fuse block.
So today I rechecked the trailer plug for 12v to the running / tail lights Nothing. I hooked a relay to the left truck tail light and tested the relay, Working. I put an inline 15A fuse in and spliced it into the 12V 40 Amp hot line that goes to the trailer, Working. Put the 12V to one side of the relay and turned the lights on and it was Working. I cut the wire from the trailer running lights behind the plug and connected it to the relay, NOTHING ?????
I took the trailer plug apart and found a bent pin inside the rear connector of the plug !!! This was factory installed and has never been apart before. It had to be that way from the factory and it must have been bent just enough that it made contact for a few year then stopped working.
Anyway, I have running lights on the trailer now, the hard way.