I had to replace the vent hose on our 2005 Ambassador. I was like you, and even pulled the unit out trying to figure out the solution. I finally realized the shelf above the unit was held in with (I think 4 screws). Removed them and the shelf lifted out. I could then access the rear of the unit with no problem.
Larry, Vickie, and Roscoe (pug)
2005 Holiday Rambler 36 PRT
2006 Chevy Malibu