You are not going to gain much if any by downsizing.
Case in point is my 26ft TT, empty is 5K, loaded with all of our gear in the truck bed plus trailer we are about 7K. A couple of years ago, we were towing with a 2006 F250 with 3V 5.4, typically we averaged about 9 MPG.
We also did nearly the same trip with our 18ft 10K flatbed, empty it weighs 2400 lbs. We got 10 MPG with the flat bed empty, on the way home with 3,500 lb load (5900 lbs)on the flatbed we averaged 9.8 MPG.
I really don't understand why the fuss about mileage, really at the end of the day you spend a few dollars more but it shouldn't be the driving force in the end.
I have said before, if you are worried about mileage, sell the RV, buy a Prius and visit Hotels and Motels...