RE: Did you ever thought about a tractor...
Ive thought about using a "tractor" as well. There's been a few german halftracks that have been sold on ebay the last couple years that are remote control. One went for $600. But they are 1/6th size scale. great for most of mine I suppose, but buying a brand new one of those half tracks is well over a couple 1000 dollars.
Another option Id thought of was using an old Willys pickup body on an r/c monster truck and painting it military colors.