One thing you didn't mention is the terrain you travel over most often. However, as it has been said, that isn't a surprise with a trailer that size. I've had a 2004 towing a 4500-5000 lb trailer and I'd get about 10 MPG. Now I have a 2017 1/2 ton GMC and our trailer is almost the same as yours. If I am traveling on flats, with our 6 speed I can get about 9, but add any inclines, even somewhat shallow and the RPMs go up (or I lose all momentum) and the MPG drops. I think I get in the PNW with hills and mountain ranges everywhere I go, about 7-8 MPG overall.
