Need advice if safe for my pickup to pull a heavy 5th wheel

I think you're flirtin with disaster.

1. Likely well over your weight ratings.
2. That old of a truck likely doesn't have added measures such as exhaust brake, tow haul mode.
3. Steep mountain passes.

I have a 2017 Ram 3500 (non Dually). My 5th wheel is teetering around 15,000 lbs. I would not be comfortable towing 16,000 with it, especially steep mountain passes.

You should be more than just a little concerned.

Mike