Mounting order should make no difference.
If you are moving the whole group use a move command instead, that might help.
The very slow moving in circles is associated with the AI trying to figure out a clear path to the destination. Even though there is actually plenty of space, nearby vehicles can cause problems. Try and get your wagons spaced out before issuing such commands.
If they are creeping and never get there, it's not impossible that it's because the doMove command has 'timed out'. Don't issue doMove commands repeatedly in an effort to make them get there - the queuing of commands just makes things worse - but an occasional reminder seems to help.
If appropriate, split a vehicle out into an existing but empty group, especially created for the purpose. You can then SetWPPos waypoints to the right place, and if necessary put the vehicle back into its original group on arrival.
If you want a loon to drive a vehicle, make him maximum skill. I don't know if it really helps or not, but it makes you feel like you are doing something.
Try issuing the command to both commander and driver.
Static objects (whether placed in the editor or, for example, burnt out vehicles) can have a remarkable effect on an AI driver. Keep such things well out of the way if you can. Never place a static object right next to a road. It can totally disrupt an AI driver even if he doesn't have to drive past it.