Trailer brakes are often unable to lock up the wheels (and thus squeal the tires). My Boler's brakes provide the trailer's share of braking with a moderate controller setting (about 7) so they clearly work properly... but they can't lock the tires even at the maximum setting.
If the trailer brakes are strong enough to do their share of braking, then with the manual lever fully applied they should be able to provide braking force equal to a large fraction of the trailer's weight. If the trailer weighs (for instance) half of the loaded truck weight, then the trailer brakes (fully manually applied) would slow down the truck plus trailer at one-third of the deceleration of a normal hard stop... that's significant braking, but not slam-you-into-the-seatbelt stuff.
Originally Posted by ChuckS
Antilock brakes might not let you squeal the brakes.
The anti-lock brake control system in the truck has no way to know the speed of the trailer wheels, so it can't control lockup of the trailer wheels. I have no idea why GM would program the integrated trailer brake controller to limit the power to the trailer brakes when the manual control is applied, and certainly not when there is lockup or control issue with the truck's brakes.
The way to objectively assess the controller's behaviour is to check the voltage supplied to the trailer brakes when the lever is applied. I would try this while safely parked, at least to start.