A lot depends on the size of the coach, are we talking a 26 ft long 15,000 GVWR chassis or a 38 ft long 24,000 GVWR chassis, also does it have the 4 speed transmission as use don the P series chassis with 8.1L Vortec, or the 5, or in later years 6 speeds used on the W series chassis? Then comes where and how are you driving, driving at 55 mph on mostly flat land I get a bit over 9 mpg on my 17,000 GVWR 28 ft coach on P32 chassis with 4 speed transmission, speed that up to 65 mph and it drops to around 8mpg, add mountains in and it goes way down. The lighter 5 and 6 speed W series coaches probably get a little better, the heavier ones get worse.
My 2002 Winnebago Adventurer 35U with the 8.1 Vortec and the W22 chassis gets between 5 and 7.5 depending on terrain, wind and whether it is in overdrive or not. Once I was on flat highway for the entire day driving at 62 mph and was disappointed to see that I only got 6 mpg but then remembered the generator was also running most of that very hot day. So in a nutshell - it will vary but doubt you will ever get over 7.5. I am happy if I get 6 consistently. I am not sure of generator fuel useage per hour but feel that it is not minimal.
Tuneup? What engine made in the last 20 years is "tuned up"?
Some serious head work, headers, intake, tweaked ECM and a different bumpstick might net you a 10% increase which sounds like a lot but going from low 7mpg and change to high 7mpg and change hardly seems worth it.
My 2004 36 foot Workhorse chassis with the 8.1 engine has averaged 7.1 MPG over the past 90,000 miles pulling a 4,000 lb toad. That's hand calculated.
Every gallon of gas I've put in it has been logged on a clipboard I keep by the drivers seat. It also serves as a trip history log...I can tell you where we were at any given time for the past 14 years.