I have a 04 RAM diesel and have fuel records from 2004 to date. Towing a 24ft, 7800lb 5th wheel, I got between 9 and 11 MPG towing in the northwest where mountains are real. I recently sold the 5th wheel and bought a 29ft TT which by chance, is also about 7880lbs with SOME stuff in it. Now I get between 8 and 10MPG with an average tow speed on the interstate around 65MPH. The tongue weight is 1000lbs. On the 2004 RAM, the factory hitch is rated for 1200lbs, in the documents I've found.
The scale operators near me, frown audibly when I tried to disconnect my trailer from the TV to get the Trailer weight, so I have to find another way to weigh.
All of this probably doesn't help answer your question though...
No matter what you tow, keep an eye on the transmission temp, it can creep up real quick when it's hot out, the A/C is on and you're pulling up hill.
Good luck with your choice and safe travels, no matter where you go..