The vehicles in a convoy will order themselves by number in the group - so 1 will be first, 2 second .....12 last. Note that which unit counts as the vehicle is different for armour (commander/gunner) and cars/lorries (driver.) (I think ... you may need to check that.)(
The group will try to organise itself into the right order before setting off, which can lead to problems if they are in an confined space like a road through a wood. You'll probably need to experiment with starting positions until you get it right.
Oh, and remember to set formation "column".
Don't use too many waypoints as they pause at each one.
If you are having trouble with tanks put a soft vehicle as the group leader. It will stick to the road. I dunno if the tanks will follow, but its worth a try.