This is pretty simple to do.

They cannot claim adverse possession if the possession was not adverse, so you just need to make it clear you are letting them rent the land with your knowledge. So draft an agreement that basically lays out the space in question and that they can lease it from you for 12 months solely for the purpose of parking their truck for the price of $1.

I mean, you should get a lawyer to do it properly. But you get the idea. As long as you make it clear that it’s your land, and you are explicitly giving them use only for some temporary period of time and specific use, they can’t claim it’s theirs. They’ve signed a statement explicitly stating they are only renting from you.