Welcome to the Airforums!
I'm not sure anyone here on the AirForums has the necessarily broad experience base to come up with even a partial list of pneumatic-suspension 6-wheel or 3-axle motorhomes— because 6-wheel motorhomes will more often be 2-axle motorhomes with dual rear wheels, and 3-axle motorhomes will typically have 8 or 10 wheels by virtue of having at least one rear axle with duals as well.
And you didn't specify whether you're only looking at Class A motorhomes or also Class C motorhomes. And if you include Sprinter-based Class B motorhomes like the Airstream Interstate (most of which have 6 wheels on 2 axles, and some of which have pneumatic rear suspension) the list gets very extensive very quickly.
Interestingly enough, the GMC motorhomes you mentioned are closer to Class Bs than Class As, because GMC built both the frame and the body, and farmed out some of the interiors to other companies while other GMCs had the interiors built at GMC as well. A true Class A leaves the automaker's facility as a frame and drivetrain, with no body at all, and then the body is added by the motorhome company. A Class C leaves the automaker's facility as a cab-and-chassis. Only a Class B leaves the automaker's facility with a full-length body already installed.
I thought getting old would take longer!