With a 2009 Ram hemi and 19ft trailer weighing about 4,000 lbs I tested the mileage on a flat section of interstate in Louisiana at three speeds with no wind: 65, 60 and 55. For each 5 mph drop in speed the mileage increased 1.5 mpg. So at 55 I was getting about 3 mpg better than at 65! Best advice is to slow down. Most of our trips the overall mileage ranged from 8 - 11 mpg.
We just upgraded to a larger RV and a 2500 Ram with the 6.4 hemi. If fuel mileage is a big deal to someone I tell them it would be best to just give up on the idea of RVing with a trailer with a lot of room.
I now tell everyone we get between "F and E" - it starts out at F and when it drops to E we add gas it goes back up to F!