We've towed a lot of different combinations of trucks, trailers and 5th wheels.
Last year, we towed a 28' 5th wheel with a Chevy 2500HD Diesel. This year, we towed a 30' 5th wheel with a Dodge 3500HD Diesel -- what a HUGE difference. The Dodge was TOTALLY the beast for towing. We went through the exact same insane mountain pass both years and the Dodge didn't even hiccup. You couldn't even tell it was towing anything, whereas with the Chev, it was working really hard. THe Dodge was just a towing rig -- had engine brakes and everything! I felt like I was in a semi, LOL.
We had no problems installing 2 carseats outboard and center in the crew area of either vehicle. LATCH & TTs in all three locations (but we are in Canada, not sure if that is the case with US trucks).
If we were going to purchase a truck for towing, we'd definitely get a 3500 Diesel. I was really impressed with the Dodge for towing, but the fit & finish on the Chev was nicer (felt less like a work truck, more like a car).
HTH