Our first tt was a Jayco 22FB (6K lbs, 27' length). We managed to average 11 mpg. Our new tt is a Sprinter (9K lbs, 32'). Our mileage is the same. As people have stated, over the long haul, weight and length don't matter much.
From experience, the smaller the trailer the more time you will spend outside.
Also from experience, having adequate clothes storage for longer trips is the hardest thing. You can always shop on the way for food so need to carry less. And, make sure you get a tow vehicle that has an adequate payload.
