Hmm, you could try to create an rotational bundle and addtemplate an spawner to it, and let that spawner spawn a tank, with holdobject set to 1.
Or.. you could try to actually program the AI to drive in tanks (create an AI spawner with that tank). But im not gonna touch that any further, since i dont know anything about the AI.
Or, put the tanks up the hills, and swet drag low, so they roll down, without any help. (put a lake below, so the blow when theyre down:P)
Or make a combination out of all 3
.
The rotational bundle i think suits best for aircraft tho. Or helo's...