Just around the tip of Alton Bay from Downing's is the free town of Alton public ramp with free auto parking right there. You will have to drive about a mile to park a trailer across from the highway dept on Letter S Road.
You can drive a boat trailer by the gas pumps at the Alton Village store as well as the Alton Circle store. They will happily take your money no matter what state you are from.
