Gary,
I disagree with probably just about everyone above (not unusual for me) ... in that I mis-implied that your rig is only "4WD". ;)
Your outstanding one-is-all-you-need Class C is indeed a 6X6 (whenever both differentials are applying power to all of their axles). The only thing that counts is how many tire treads are pulling on Mother Earth to provide traction. You have six (6) tires doing this, just like with the big Army rigs ... hence "6X6" applies to you.
Your only difference from the Army rigs is you have to be careful off-highway to NOT GET just the right sized sharp rocks stuck in between you rear duals, which is not a concern with tandem axles. HOWEVER ... I'll bet that on soft surfaces side-by-side tires in dually sets will tend to sink in less than will tandem tires that are following each other in the same rut.
One is all you need .... buy the right one once .... and keep on truckin' without getting stuckin'. :B and :C