Wow!, Can't believe no one else chimed in, usually fuel mileage posts get a lot of opinions.
Wanted to update my actual findings, just returned from a 2755 mile trip running just the RV, no trailer thru 7 states with various mountain grades. I hand calculate every tank. I usually run 400+ miles between fill up. I don't like to run much under a half tank, and when I took this trip it was during the east coast fuel shortage so I didn't want to go too low.
I averaged 65-70 with a fully loaded coach, full fuel and full water, this was a 3 week vacation, worst tank was 7.5 mpg, best was 10, over the entire trip actual fuel mileage was 9.0. I ran 75+ down in FL and on flat highway's. Power in the hills was noticable and downshifting was greatly reduced.
Having owned 44 motorhomes over the last 35 years, I am extremely pleased with this ECM update, and every tank was with 87 octane fuel, I told them when I purchased it that I did NOT want to have to run premium fuel.
It definitely makes a difference.
My coach is a 2005 Gulfstream Endura model 6316, started the trip with 44,000 actual miles, new tires, fresh tune-up, minimal generator operation while traveling.