Thinking future 5th wheel = 3500 of whatever brand you prefer. Diesel will pull better, especially in mountains and will have better overall mpg. The added cost will likely pay for itself with the amount of mileage you drive. I only put on about 12k/yr and tow a <8k trailer 3-4 times a year. I got the diesel because it was a no cost option at the time ($9k value) so the cost was the same as the gas truck. Oil and filter changes are way more expensive but I get much better mpg than my old gas truck and diesel fuel is cheaper than regular where I live so I come out ahead.
