2021 Sierra Trailer Brake Problems

[Whip]

I am still struggling trying to diagnose a problem with my trailer brake controller. My post is long, but I'm hoping someone with more knowledge than me might be able to deduce what is going on.  I have read about the issues with 2024 trucks when connected to a trailer with solar.  That sure sounds similar to what I've been experiencing, but doesn't seem like it applies to older trucks.  So I'm starting this thread from scratch.

 

Very sporadically while towing an RV I get the message "Check trailer wiring" followed immediately by "Trailer brakes connected". Sometimes it doesn't happen for a copulation hundred miles, and then will happen repeatedly ten times in a row before stopping again.

Twice I've gotten a message saying "Service trailer brake system". When this battlements I lose the trailer brakes entirely and find that the fuse for the trailer brakes is blown. I replace the fuse and everything works again for awhile.

 

I towed a different trailer that did not have trailer brakes and with that one I got the "Check trailer wiring" message and found that I had lost the rear running lights and side marker lights. After unplugging, reconnecting and restarting the truck they were back on.

 

I have replaced the trailer brake controller located above the spare tire and also the trailer connector plug in on the truck.

My truck is a 2021 GMC Sierra 2500 HD.

 

I'm at a loss on what to try next or how to diagnose this.  Any help is appreciated!