I stayed at the Miccosukee Casino, just west of Miami. It's just a parking lot but free. Have to go in and register but not a hassle. There were probably 12 rv'ers there that night.
Side note, joined the players club and got $20 in slot play and walked out with $45. The buffet is not bad but nothing special.
Edit: If you go here use the Northwest corner of their parking lot. It's the area designated for RV's. Some trucks will be there too.