I am currently moving from CA to CO and yesterday while towing a 6% grade the truck started slipping we were near the top of the pass holding at around 65mph. My wife was driving while I was sleeping but the surging woke me up. I had her drop the trans to 5 gear and manually to 4 on the next steep pull. The truck is a 2017 with 60k running mm3 from drd stock otherwise. The trailer is probably close to 10k lb. I dropped the tune back to stock the rest of the day and locked out 6th . It didn't seem to have the issue again. I have never run the truck above tow tune even empty.
Any recommendations including a good shop in Colorado on the Colorado springs side of things, should I look at a revmax converter and valve body or other option.
Thanks for the help