[bobkatfly]

In Tuftonboro, across from the town beach on rt 109, there is a large parking lot. It's used by vehicles w/trailers that launch boats from the Pier 19 ramp (not the best ramp around but still a ramp). The lot is a very short drive from the ramp. With that said, I'm not certain about the extended stay limitations there, especially with three vehicles. I've seen pickups with trailers overnight from time to time though. There's an association that owns the boat slips at the ramp but I don't think it belongs to them. Seems to be a lot for overflow at the beach. Maybe someone else here can chime in about that. Otherwise, you can call the Tuftonboro police dept to inquire. Maybe you can get a short term permit.
More importantly, pay attention to what pricestavern and TiltonBB said about navigating the lake if you're not familiar with it.