I guess I don't understand. The Montana website
http://keystone-montana.com/index.ph...pecs&year=2008 (for 2008 but the rest of the years can be found from here) shows the GVWR to be 15,480 (11,920 Shipping Weight + 3,560 Carrying Capacity).
The numbers for 2009 are 12,032 + 3,448 = 15,480
And the numbers for 2007 are 11,685 + 3,735 = 15,420
It appears the 2008 and the years before and after are all rated at 15K GVWR. This doesn't sound like an axle change from 6K to 7K occurred in these years as the GVWR and the Carrying Capacity are very close.
What sticker are you looking at? Check the one near the propane locker on the driver's side of the RV and the one inside the kitchen cabinet. They should correspond with similar numbers. The propane locker one will tell you which axles are installed.
Is it possible you are looking at the kitchen cabinet door sticker and it had been replaced by a door from another rig?