Hi Pnichols, On the Chev 8.1 liter based motorhome the gas does appear to be shut off to injectors when going downhill the scan gauge would read 99999 mpg. However on the Ford 6.8 liter engine my scan gauge reads 45 to 49 mpg going down hill...fuel does not appear to be shut off to the injectors. So mileage does suffer in mountain driving.
We get 9.0 to 9.3 mpg on long trips with a Ford E450 based Motorhome (26ft). This is based on mileage divided by fuel used .....not by the scan gauge numbers.