Your uncle probably cannot charge you in this case. If he had, at the outset, said you'd have to pay some storage cost or fee, that would be legal. Or if you put the truck on his property without his permission, he may have grounds to seek compensation. But from what you write, your uncle had given you permission to park the truck there without having to pay anything. He may not after the fact change the agreement or offer to now require payment. If he will not let you have the truck back without paying, you may need to bring a legal action and seek a court order directing him to turn the truck over to you, however.