I would think of it like there is a global, practically inexhaustable truck pool. When you create a supply vehicle, you're really just creating the supplies, then demanding a truck come and pick it up (something we can assume is a trivial matter for an interstellar empire to organise). They can be comandeered civilian trucks, purpose built trucks, whatever, they spend most of their time existing in the aether of the society until they are required. Then, when their supplies have been exhausted they go and do things, get maintained, make their way to where they need to go by civilian means, whatever, melt back into society and may or may not be one of the future comandeered trucks.
Yes, it's a little abstract, but it makes more sense than the trucks just spontaneously combusting because they ran out of bullets or something, to my mind.