Yes he can get that mileage just before he wakes up from the dream that he's having! Even a coach that does get an occasional 10 mpg won't do it at 75 mph.
Here's what typically happens.....you have guys that never really tract their mileage, but will on occasion see that they didn't take on as much fuel as they thought they would at a particular fuel stop. They check their mileage and see they got about 9.8 and round it off to 10 mpg. What really happened was they were on an angle at the last fuel up and added an additional 5 gallons of fuel. Then they fill up angled the other way and take 5-7 gallons less than they normally would. So between the two fill ups they were off by over 10 gallons. If you drive 400 miles and take on 40 gallons you get 10 mpg. Now add in that 10 gallons of over and under fueling and you're now at 8 mpg.
The above happens often with the DP's since the tanks are almost 8' across and leaning in one direction or the other can really change how much they take. The Monaco tanks have metal pipe fillers welded on the top edge of both sides of the tank and are really susceptible to this.
We just returned from a 6300 mile trip. We started the trip and ended the trip by fueling up at the same station and in the same position. We then calculated the fuel mileage for the entire trip. We saw some 7 mpg's and some 11 mpg's, all caused by how the coach was leaning when we fueled up. We have a short and fairly light DP with a very economical ISL 400. We averaged 9.24 mpg over 6000 miles towing our truck at 58-59 mph (we only passed two other RV's). I can definitely say your friend driving 75 mph did not get 10 mpg.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab
