I guess I dont understand the garage issue.....both 3/4 and 1/2 ton trucks are the same size in the GM and Dodge lines, assuming we are looking at the same body style..shoet bed, crew cab, etc.
Anyway, I Have owned two GM HD 2500s, and they are certainly hyge gas guzzelers....less tahn 13 hiway unloade and 6-7 towing 8000 pounds.
I would guess your 7000 pound trailer weighs a lot more when loaded for a trip, and I would prefer a 3/4 ton to pull that load.....the 3/4 has a more stable towing ability, bigger brakes, heavier suspension, etc. A much better towing experience, IMO.
My Ram 2500 Hemi gets 9-10 mpg towing and 19-21 hiway empty....rides better than any 3/4 ton I have ever owned, but it is 2wd.
I doubt that you can have too much truck!