Looking to get an 03 Chevy 2500hd duramax I want to make sure it's going to be okay towing a 35 foot 2 car goose neck and also what you guys think the mpg is when the trailer is loaded?

Loaded and gas mileage should not be discussed....
You will like the duramax on the hook

9-13 mpg depending on load/wind/speed etc.

You'll like that truck a lot. Go over to duramaxdiesels dot com and ask your questions.