honest advice on a truck for towing 10000lbs

I am looking to get a sandstorm toy hauler travel trailer. I am wanting some advice on the tow vehicle. I am neutral when it comes to brand. The trailer will be 10k or less gvwr. I have the following questions. Is it worth it to get a diesel factoring in the cost of fuel and mileage. What truck is best for price. I would like to know real world mileage. I need a min of 10mpg if possible. I will be towing in various terrain and on trips over 1000miles at least once a year. Most of the towing will be within 200miles to the dunes. I want to spend under 30k on the vehicle. I don't mind getting a used vehicle especially if the newdiesels are not that great. I am strongly looking at a 2500 Silverado with the duramax. But I was wondering if the diesel upgrade is even worth it. 9000 difference in price buys alot of gas if the towing fuel mileage is not a huge difference. Thanks for the advice.