Rather than rely on some negative opinions, the easy way to find out what the OP (Charlie) will do is to email him at the address he provided. Only the OP knows what and who they will accept. I will bet that Charlie will not view this as "work" per se, in the sense of needing a green card, because you would not receive any pay whatsoever. You'd be bartering your time for a reduced camping rate. With no pay stubs or payment paper trail, I think the government regulations are out of the picture with no one the wiser.
Don't let anyone rain on your parade, Peter. Follow your dreams until you find the way through.
To everything... there is a season...
and a time to every purpose under heaven... --The Byrds