Diesel is where it's at for towing.
Gas trucks are good for near-home utility, but for interstate towing - there's no comparison. Sure, you can tow with a gas truck, but I can tow with my limo too. It doesn't mean it's going to be pleasant.
Everybody has an opinion, but the Cummins in @Robert M's Ram is near bulletproof, and has been since mastodons roamed the earth.
The Duramax diesels in GM trucks are reputed to be fantastic - right-hand man Brad just got a used Duramax crew HD2500, and is now getting 14 mpg towing his 28 ft bumper-pull camper (after getting 7 mpg with his ecoboost).
I've got a love/hate with the Ford Powerstrokes like @DannyP is considering. I had two 7.3's and loved them both... except when the o-rings in the oil-pressure driven injectors started leaking and required $2000 worth of work. They seemed to take more work than the Rams or Duramaxes did to stay running. The 6.0s have a pretty bad reputation - it was Ford's first swing at an aluminum diesel (which is kind've an answer in search of a question), and they're widely reputed to be nowhere near as reliable and powerful as the 7.3 they replaced. The 6.7 fixed a lot of the 6.0's issues, but the damage is done - a similarly equipped Ram with the Cummins is tens of thousands of dollars less (purchased new), but costs less to operate and holds its value better.
Of course, buying used - all bets are off, as the value equation gets skewed. A used 250/2500 series diesel truck from various manufacturers depreciate at different rates (except for the 6.0 Fords, which are many, many thousands cheaper), and the value equation changes. Rams have a tendency to rot away in the rust-belt but their engines run forever. Fords are built beautifully, but the 6.0 was a pretty big stain on their reputation. Chevy/GMC Duramaxes hit the sweet-spot, but good luck finding one that isn't terribly overpriced.
Regardless, the days of being a "Ford guy" or a "Mopar guy" are long since past. Any of these trucks (6.0 Powerstroke possibly excluded) will tick every box and all of them (including the 6.0 Powerstroke) will be worlds better than any gas truck (V10 included) towing long distances.