These types of questions without a lot of info on future plans, location (weather, urban rural, mountainous,) make it really difficult for anyone to really make an informed suggestion.
If you had no plans to update in the time you will own the truck, and want something easier for to drive in town, a gas Crew cab Colorado could pull your trailer with room to spare. The I-4 Duramax has the same amount of torque as a GMT 400 454 and falls about 50 hp of matching the big block in hp.
You want to increase your trailer somewhat in the future, but don't plan on going a lot larger. A 1500 depending on the specs will get you to about 12K. gas or diesel is now or will be very soon an option with all three prime OEMs.
If you plan adding a substantially larger RV within the time you plan on owning this vehicle than get a HD. Gas or diesel? Really depends on your money situation, distance traveled, topography of where you would go RVing and host of there factors.
Then there are factors that simply come down to what you want (assuming your pockets are deep enough to satisfy your "want").
I know many guys who drive an HD simply because that is what they want to drive. They don't pull anything, but they wont buy a 1500. Diesel is the same way, many guys simply want to drive a diesel, even though a gas would easily satisfy their towing needs. Long box, short box, it goes on and on.
Good luck, let us know where you end up.
