The night to book in Anza-Borrego is Borrego Palm Canyon, and it only exists from October through April. The developed campgrounds take reservations across that window, released six months ahead on ReserveCalifornia (parks.ca.gov). Lock the site first and go find a van second, because the campground calendar is the part you cannot move.
California's largest state park starts at the city's back edge and runs out across palm canyons, badlands, and hundreds of miles of dirt road. The walk most people do is Borrego Palm Canyon, and the palm grove at the end of it is closed, so the trail turns back at an overlook of the post-fire recovery (parks.ca.gov).
If the loop is full when you get there, pull off along a designated dirt road and camp for free, no more than one car length off the pavement, with fires only in a metal container you carry in (parks.ca.gov).
Font's Point is the one to leave alone. The badlands overlook everyone photographs sits at the end of a deep-sand road that stays soft in the S-curves even with four-wheel drive, and a rental camper has no business out there. The Slot, off Highway 78, is the dirt spur a two-wheel-drive van can actually manage, a sandstone corridor narrow enough to touch both walls (parks.ca.gov).
Then summer arrives, the desert campgrounds close, and the whole trip moves to the coast.
San Elijo State Beach, Cardiff
San Elijo is where most San Diego renters take the summer night, a row of bluff-top sites on Coast Highway 101 in Cardiff with stairs cut down the cliff to a narrow surf beach. It takes tents as well as hookups, so a conversion van with no toilet still gets a legal night on the water. Hookup sites cap at 24 feet and the dry sites at 35 (parks.ca.gov), so if you are sizing up past a van, RV rentals in San Diego fit the same rows.
Just south, Torrey Pines State Natural Reserve is the day hike, rare wind-bent pines on broken cliffs over the ocean, and there is no camping in the reserve, so you drive back down the coast to sleep (parks.ca.gov). Head north on 101 instead and the next bluff campground is South Carlsbad, which belongs to the Carlsbad campervan trip, and the California campervan hub picks up the coast north and south of here.
Silver Strand State Beach, Coronado
Silver Strand is the in-city first night, a sand spit 4.5 miles south of Coronado on Highway 75 with the Pacific on one side and San Diego Bay on the other. The catch is at the kiosk. The state wants running water and a flushable toilet with a holding tank, or a self-contained toilet, and a van that fails the check is turned away (parks.ca.gov). Sites are hookup only, so pick the hook-up type when you reserve.
If the van does not qualify or the state beaches are booked out, the private waterfront pads at Campland or Mission Bay RV Resort take the overflow, though neither is the reason you rented a camper.