Yep, same thing happened to us this last Fall when we were heading to Michigan. I got beat up pretty bad on these forums for towing without brakes on the trailer. But, you've got to do what you've got to do, at the time you have to do it. No apologies for doing that.
However, WHEN we finally got the truck into a repair shop, it turned out to be the trailer brake relay switch.
About $250 for parts, service, and all fees, and a hour of work and we were were on our way again. We did take the truck to a Chevy dealership repair shop (Marquette, Michigan). Haven't had problems since.
And yes, it displayed with the trailer connected and without the trailer connected. It's your trailer brake relay switch. I almost guarantee it.
( I saw your same post on RV.net, but I'm not responding over there).
To plant a garden is to believe in tomorrow.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually