Who's worried about a typical steepest it can be 6% interstate grade?!?!? Not me. Be it a gaz or deezal!
There are some 15% grades in the NE corner of Yellowstone....that can be interesting.
Otherwise, I haven't had but one rig that would not hold me on a downhill. That was my 88 SW K3500 where the POS th400 lost the ability to shift to 2nd gear. So you went 60-70 mph down typical 4-6% interstate grades, or 25-30 no brakes in 1st. It would hold 15k lbs in 2nd before loosing that gear. My 89 R3500 did that too.
For what it's worth, my 14 4.3 V6 is every bit as good or better pulling my equipment trailer at 6500 as those to supposed mountain climbing 454's were.
As Grit points out, as long as your on a typical freeway grade, the new rigs be them gaz or deezal, will be fine.
Marty