Please help me identify 5th wheel towing issue.

You gave values of 12.5k# and 2,100# pin. Are those the dry weights from the manufacturers brochure, or did you actually weight your rig.

Assuming those are your weights while towing, you are very light on pin weight. And I think the fact that it is a problem while angled up a hill, supports my theory.

Experiment with increasing your pin weight and see if that helps.

Your numbers are a 17% pin weight. Your goal should be more like 25%. Or at worst 20%.