geof, is that cab a air cab?
From what I understand, on none air cabs (vehicles in the class C size) the box and the cab are fitted very close and a interior seal can be used.
On rig's with air cab there is about 5"'s of movement and the space between box and the cab must be much be further apart and they use a very large seal. This presents a bigger opportunity for problems. I know it is being done, but most companies are eliminating this potential problem.
Dan, your only about 3 hours away, stop by sometime and I can show you around the rig. The gearmaster unit is awesome and a must for anyone with a manual and not running it all of the time or has a bunch of years experience. On my trip to and from Florida a few weeks ago I used that thing a ton when I found myself in major traffic jams. Speeds where jumping all over and I was having to downshift so much and this devise puts you in the right gear.
2003 28' Show Hauler Motorhome on a 1995 FLD 120
www.showhauler.com