Hi Dave,
There are lots of us building/flying RVs around Boston. The choice of airports will depend on where you plan to live.
If you'll be on the north/northwest side of town, there are several controlled fields - Beverly, Lawrence, Hanscom and Nashua NH - and a couple of uncontrolled fields - Minuteman and Fitchburg. Lawrence has the most active EAA chapter in the area - Chapter 106.
On the south side, the only controlled field is Norwood, uncontrolled fields are Marshfield, Taunton, Plymouth, and Mansfield.
Where to go? If I were moving to the area and could make the commute work, I'd go to Plymouth. There are already several RVs either flying or building around that area, there's a good EAA chapter at the airport, and hangar space can usually be found. The only place I'd avoid is Hanscom. I teach out of there now - too busy, too expensive, too many bizjets, hangar space hard to find and expensive.
Good luck with your move, I'll be glad to give you more info on or off-list...shoot me a PM or email (setser at rcn dot com).
Dave