I'll get in and correct myself , before someone jumps all over me.
Never trust anything to memory.
Look for the PDF file from Cat. " Understanding Coach / RV Performance "
I have it in print but I think the PDF , went south with an old computer .
3 biggest factors in fuel economy , RV.
Driver, best driver to worst a 20% difference.
Route, interstate to congested surface roads, 20 % difference.
Vehicle speed, 60 mph Vs 70 mph, 12 % difference.
Idle time is listed as a factor, 20 % idle to total run time vs 10 % , but only rated at a 1.5 % difference.
My own coach lifetime MPG shows as 6.8, yet on highway regularly gets 9+.
The PDF file used to be available on the Cat home page, but disappeared when Cat went out of the on highway engine business.
Maybe someone in the forums still has it and can get it for you.