Welcome to the forum.
In general you'll want to engage the tow/haul mode whenever you're towing / loaded. It will adjust the Alli's shift points, up and down to accommodate your increased weight.
Also energize engine braking it will cause the down shifts to be more aggressive taking some of the load off of your braking systems. Both truck and trailer.
This works very well on long grades that can and will overheat brake systems on gasoline (lower compression) tow vehicles.
The engine braking primarily uses the engines high compression drag as reverse (BHP) brake horsepower. The higher the RPM the more BHP. It also uses the control vanes incorporated into the turbocharger to restrict this internal air flow.
Granted EB'ing will help reduce brake wear while driving the empty truck, but we find it a little to aggressive jerky when not towing / loaded. Also not desirable in limited traction conditions with a light rear-end.
Yes applying the brakes decelerating on a decent will "usually" cause a downshift, but it mainly depends on the engine RPM, speed, and other function being monitored by the ECM & TCM. Those modules will protect the engine from over revving, some more brake could be required. Remember though the max. engine braking force is at a higher RPMs. This can take a little getting used to. With some hrs. in the saddle you'll realize that the cruise control (if equipped) does a pretty darn good job of controlling that D-Max / Alli on most single digit% grade decents. We travel most of the western passes towing our nearly 20K# 5'r on cruise. GM has done a great job refining these control modules.
We tend to try and crest a hill / pass at the speed we'd find comfortable during the decent, say slightly below the speed limit. this will generally require less brake pedal applications.
engine braking on
Enjoy & Happy trails.
J & J, DRV Suites ES-38RSSA #9679
GM Denali, 3500HD-Max, 4x CC, 8' DRW,
EZGo-TXT, Clubcar Precedent