A truck and a hybrid camper, you've got more options and I wouldn't limit yourself to an MBTA parking lot or unhitching at an RV park.
Fenway, USS Constitution have oversize vehicle / tour bus lots you could probably get into. Or you could probably just park on street in the seaport/world trade center area. Maybe Assembly Row or the Home Depot near it.
Of course your rig won't be much bigger than the u-haul/ryder truck rentals that get double parked all over town before labor day, so you might be able to just blend in.