I saw a 6 x 6 (driving front wheels) on a Freightliner class 8 chassis at Showhauler last fall. It was bein built out for a guy who was really into hunting and wanted to be able to get into the "back country" with his rig. Personally, I'd have some revervations about "off roading" with a 36 foot motorhome, but to each his own!
I know of two places that have this sort of system. The links are:
http://www.allwheeldriveequip.com/
http://www.meritorhvs.com/
There may be more places, but I've not heard of them.
Hope this helps!