MPG for Various Class A Motorhomes

I have a 1996 GBM with the 454 TBI 33 ft weighing 15,000 lbs and have averaged 8.6 MPG over 100k miles of travel. I tow a 2500 lb car and don't see any difference in towing or not towing. I dry camp 95% of the time but use the genset very little. I drive at 62 mph mostly on the freeway. I do better on secondary roads in 4th gear between 45 - 55 MPH(over 9mpg). Tried lots of things to improve performance and MPG but adding headers, FF muffler, CAI and advancing the timing all added performance but no mpg increase. Adding a vacuum gauge and keeping vacuum over 10 in of hg will add about .25 mpg if I follow it. Some on here have seen mpg increase with these types of improvements but not me. Slowing down is the only mpg increase that I have seen. My 3 friends with DPs all get less mpg because they are twice my weight. I would be interested to see what small class A's or C's average.