The truck is a 2011 GMC Sierra Denali 2500HD crew cab with 6 bed.
changed from a 26' 5th wheel to a 24' regular trailer
that truck should stop a 24' trailer just fine. take the unit and go find a large parking lot that is not being used after hours, on a day the business is closed etc.
you will need a big enough lot to get a little speed up, but you don't need to be on a race course. see if you can find the minimum speed the problem occurs at.
then unplug the trailer and see how the truck stops.
if the problem is still present, it is in your truck (or induced into your truck by the trailer being hitched to it). if the problem has gone away, it is the trailer and you can then go from there.
