I bought a new fifth wheel last week, and it weighs 12k lbs. with 2K on the Pin dry. My 2003 F250 7.3 with a 60 hp tow mode on a D-P Tuner pulls it just fine. When the trailer's dropped on the B&W Patriot hitch, the rear drops about 2"--as expected. My new AirRide airbags are kept at a minimum 11 lbs. of air pressure.
But I carefully chose my Grand Design 323BHS as the largest trailer I'd ever tow with single rear wheels. And we're not suffering a bit with our truck. But any truck carrying such weights needs to be properly prepared, and mechanics, brakes, suspension, shocks need to be kept 100% maintained.