9-10mpg is quite good for any high walled RV pulled by a gas engine. You may get 11-12mpg with any diesel, the Ram 1500 3.0L or Ram 3500 6.7L. Are your towing miles that significant to justify? 9mpg vs. 12mpg @ $4/gal for either fuel towing 5000 miles saves $555.
If the trans is shifting a lot by whatever definition you have, then lock out the highest gear or two. Also use tow/haul mode. Preventing the transmission from using too high a gear will provide less shifting nuisance, better throttle response, and possibly use less fuel. At 65mph on flat highway, I recommend keeping your engine in the 2500-3000rpm range.