We started off with an 18 foot hybrid travel trailer, about 3500lb towed with a minivan. We saw about 13-14 mpg when towing.
Bought an F-150 in preparation for getting a larger trailer, and saw 9-10 mpg towing that same 3500lb travel trailer. The minivan did a much better job with regard to fuel economy towing than did the truck. When we bought the larger 6000lb trailer I also see the same 9-10 mpg towing with the truck.
So I would agree the mileage you see when towing won't be appreciably different with different size travel trailers, IMO the choice of tow vehicle has a larger effect here. However like you say a big concern is the mileage you see when NOT towing. Our minivan got 30mpg on the highway while the F-150 was lucky to see 20mpg hwy, and 12mpg in town! Its hard to say what the 'sweet spot' is other than
- towing mpg will be bad for any vehicle towing any full-height travel trailer
- if daily driving mpg is your primary concern, pick the tow vehicle with the largest tow capacity whose fuel economy you can tolerate when NOT towing
- limit your travel trailer choices to the weight range supported by your tow vehicle. Length has little effect on mpg when towing
