So, with no fare wage, they come to work for you for room and board, and at the end of the term, they have no money saved to move on with.
I would offer a fare wage for their labors, charge them a minimum amount for room and board, have them do "extras" (mending, cleaning) etc for extra wage and let them save what they could to move on with later.
I would not use my new camper for the room, but would have an older, functional one for them to rent cheaply and live in if so desired. That way your new camper would not become a total mess when you decided to use it. One man's comfortable, is not always the way I would want it to be kept, so with an older unit, you could be a little more lax in the daily policing of the unit.
Offer some incentives for other things to do around the place so they can feel useful and feel that they are earning their keep and then some, cause if they were not, they would be a burden to you rather than a help.
Think it over very carefully and good luck.
"Sometimes I just sit and think. Sometimes I just sit." "Great minds like a think."