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.