This all depends on how bad the owner is. If he's taking money from you, I'd get out immediately. If the situation is like themagicoen's give him a week or two notice. But if the owner wants it back immediately, accommodate him. It's his truck, and if he doesn't want to live by the rules HE set, so be it.
Make sure you get back everything you have coming. If you're in NC, and he wants you to dh the truck back to OH immediately, it's HIS fuel, not yours. Make sure he checks the truck out when you drop it off. Don't just leave it, then have him notify you of the broken whatsit when it comes time to pay you. Might be a good idea to get something in writing, stating the truck is in good order.
Don't be arguementative when you drop it off. If the conversation turns such, leave. Don't give him fuel to pennypinch YOUR check, just cause he has the power to. You need to be level-headed until you get your money. Then, if he owes you more, persue it.