Look up Suffolk county parks. Go to Shinnecock campground. If you have a 4x4 you can camp right on the beach. No hook ups but if you like the beach it's worth it. They have a dump station and hoses to air up on the way out.
Eagle Super lite 284bhs
2013 Tundra crew max
We used to camp at Wildwood State park when we lived on LI but only tent camped but they do have TT spots. Its a short walk to the beach. Just don't leave your coolers out at night as they have some really fat racoons !!!!