That sounds like time controlled. Proportional will brake harder the harder you hit the brakes on the truck. The one time I rode with a guy that had time control it was always rough under longer, lighter braking (in town with stop and go). It got "grabby" unless you were constantly adjusting it.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition