Well it works for me.
If you put a fresh waypoint for your tank and only touch the behaviour setting by putting it to safe I am quite sure your tank will drive on the road.
If there are enemies nearby, the tank won't behave like we want.
EDIT:
I don't see any point in giving more waypoints. I just tested this with one waypoint and it works. There's even a junction and the tank drives it somewhat right.
The amount of waypoints should be minimized. If you are going to make the tank drive a long way to it's destination, you would need a lot of additional waypoints to fix this, and it is still not guaranteed to keep on the road.
When there are enemies nearby, the tank will of course go crazy.
Your trucks will also take those little shortcuts, believe me. And imho, there's no point in making a complex script just to make the vehicles stay exactly on the road.