By midsummer the Sandbanks ridge is hot underfoot, marram grass on pale sand above Outlet, Cedars, and West Lake. You drop from that dune onto open Lake Ontario and swim, then sleep in the trees behind the beach. Prince Edward County and downtown Picton are the day off the water, wineries and a small main street behind the dunes.
Ontario Parks calls the ridge the world's largest baymouth barrier dune formation. Five campgrounds take the overnight in the trees behind the beaches, Outlet River A and B, Cedars, Richardson's, and Woodlands, and they take tents to large trailers. You stay in those trees so the beach is still there in the morning.
Ontario Parks lists Woodlands as fully electrical and Richardson's as having electrical sites. Book one of those if you want power at the pad. A tent site and an electrical site are different nights, so the overnight is the first reservation.
Ontario Parks takes reservations up to five months ahead of arrival. Summer is the crush on those beach loops, and they fill inside that window. Lock the overnight first, and treat the van as the second booking.
If Sandbanks is already gone, Presqu'ile is the next Lake Ontario night.
Presqu'ile Provincial Park
Spring and fall, Presqu'ile fills with people who came for the migration. You camp so you can be on the marsh early. You walk a marsh boardwalk while gulls and shorebirds work the shoreline, the lighthouse out past the trees.
High Bluff and Pines are the electrical loops Ontario Parks says are popular with RVs. Maples, Lakeside, Craigs, Hidden Valley, and Elmvale run without hookups. Car camping holds late April through October.
West along the same lake, Oshawa is the Darlington overnight.
Bon Echo Provincial Park
The pictographs at Mazinaw Rock are painted at the waterline on granite, so the paddle along the face is the way to see them, and the night is on Mazinaw Lake.
Ontario Parks splits that overnight in two. Mazinaw Lake's three loops, Sawmill Bay, Midway, and Fairway, take tents to large trailers, with electrical hookups on many sites. Hardwood Hill is maple-beech forest for tents and tent-trailers only, with no electrical hookups, so a campervan sleeps at Mazinaw Lake.
If you need more sleeping space than a campervan, Mazinaw Lake takes large trailers, so look at RVs for rent in Belleville.
The Kawartha Highlands and Warsaw Caves overnight is a Peterborough trip.
Toward the Thousand Islands, you pass Kingston.