First, as you have learned, never, ever believe what an rv salesman tells you!
Second, never, ever, believe what an rv salesman tells you!
Towing, expect between 6 and 8 mpg. Maybe a bit more, sometimes a bit less.
No "magic" tricks, no "add on's" will get you more.
Your driving a 6 ton box, towing 3500 lbs thru the air at 60-65 mph. Not too much you can do.
Two things I am planning on trying should get my Lexi better milage...
First, I'm going to try to make the cabin as airtight as I can, and before a trip, fill the cabin completely with helium. This should lighten the weight being transferred to the ground, thus better mileage.
Of course, fuel savings will be cancelled out by the cost of helium.
Second attempt will be to have my better half standing in the middle of the cabin, and jump up and down. I figure if she's in the air for approx. 1/2 second, during that 1/2 second, the rig will be 120 lbs lighter, thus saving gas, thus, better mileage
Over a trip time of 6 hours per day, she'll be in the air for 2 1/2 to 2 3/4 hours (have to give her a break now and then). During that (air) time, I figure the savings will be significant.
And, I will add, don't worry about mpg, go out and enjoy your rig for the reasons you bought it. Go out and see the sights, have fun, have family time, make some memories.