My tow vehicle is a GMC Denali 1500
crew cab, 4x4. It has the 5.3 V8 and an 8-speed transmission. I find that it does very well in Colorado with my 23 FB, even on steep grades. The engine provides maximum torque around 4,100 RPM, so manually shifting and keeping the engine in the optimal power band makes a big difference on steep grades. Coming down a steep grade, a gasoline engine will certainly help with engine braking, but nothing like a diesel engine. It’s important to use a combination of downshifting and brakes to keep speed under control. Fortunately, the new 1500 series trucks are now available with a smaller version of the Duramax diesel (smaller compared to the HD trucks) that seems like a nice option.
Based on my experience in the mountains, I would personally opt for an HD truck for a 25-foot or bigger trailer. I think you can make a 1500 series truck work with a 25-footer, but it’s not something I’d enjoy based on how I like to tow/camp. For a trailer that size or bigger, I’d buy a 2500 series truck with a Duramax diesel.
That’s my advice based on my personal experience. Remember, the advice you get on web forums is worth exactly what you pay for it. It’s up to you to decide what rig and what truck work for you.