In a southern German church record from 1793 I found the professional designation "Wagenknecht". From the context it is clear that the person was working in the military at the time, and it seems he was responsible for the carts and carriages of an artillery regiment. How would you translate his occupation into English?
I think the closest translation is likely: "artillery carriage driver" or just "coachman"
Before Napoleon re-organized the artillery, the gun carriages and horses were looked after by civilian contractors. They would ride the guns into position before battle commenced and would then get well away from the action. This made it difficult to reposition guns. Or they would be obliged to stay, but would often panic in the heat of battle and run away.
Napoleon introduced the modern artillery train with soldier drivers. These were generally former cavalry men, who were somehow unfit for service otherwise. These reforms were introduced in the French army in 1800.
So your ancestor seven years before that would likely just be a civilian contractor moving artillery pieces around for the army. Since these men are almost always overlooked in statistics or strength reports, it is very hard to find a good description for them. I would hazard a guess that even at the time there were many conflicting descriptions for them.
See here for more information about artillery trains.