Not enough payload for 5th wheel. I would expect you to be 1500+ pounds over GVWR, and possibly over rear axle. That 12000 pound 5er will grow to 14000 pounds or more full timing with 2500+ pounds of pin weight. Add a 200 pound receiver and some people and gear in the cab and bed. You'll need closer to 3500 pounds of payload to be comfortable.
8700 pound dry TT will grow to over 10000 pounds with 1300-1400 pounds of loaded tongue weight. Add a bed of full timing gear and people in the cab and you'll want at least 2000 pounds of payload for that TT.
Check your tire and loading sticker on driver's door for as equipped payload.
You have lots of power to pull 25k pounds over the mountains.