Are you sure there isn't a selector switch somewhere that determines the highest gear the transmission will use?
Our 2007 Itasca on the W24 chassis has a dash mounted 3-position selector switch that will determine if 4th, 5th, or 6th gear is the highest gear the transmission will shift up to. Both 5th and 6th gears are overdrive. This selector switch can be very useful in hilly or mountainous terrain. For normal, mostly level driving, we leave the switch so that the trans will go all the way up to 6th gear.
From your description, it certainly sounds like the transmission is now going all the way to 6th gear which makes a big difference on normal highways. If you do not have a high gear selector switch, I'm sure an authorized Allison center could install one for you. It seems really odd to me that the coach builder would build a mh that would not have the ability to use the transmission as built by the chassis builder.
Good luck and enjoy the 6th gear.
