Why not a "0" down.

Working toward equity is a good thing.  Using down payments of less than 10% makes it difficult to get into an equity position and increases risk because; with time often the trucks maintenance costs will go up beyond its ability to maintain a positive cash flow.

... Can't make the payment because the truck is in the shop.  A truck in the shop takes away from available paid utilization hours.  Meanwhile, shop labor and parts bills must be paid before the truck can get a paying load.

The earlier a truck owner "pay his dues" by learning good habits, learning the nuances of the profitable trucking market niches and gaining equity, the earlier he or she becomes the "bear in the woods" as opposed to the "bear in the zoo".  They can then pretty much do what they want, when they want.