Drive up the Santa Ana Canyon on the east edge of Anaheim and you reach Featherly Regional Park, where the river runs through the wooded canyon bottom and a paved bikeway follows it. You sleep at Canyon RV Park and pick up the Santa Ana River bikeway from inside the gate. Almost all of the rest of Featherly is protected riparian wilderness the public views from the trail rather than walks into, and OC Parks names the RV concession as the park's only developed piece, so the ride is how you see the river.
Canyon RV Park runs full-hookup sites with 30- and 50-amp power and takes rigs up to 45 feet, open every day of the year including holidays. The canyon sits inland of the coast's marine layer, so summer nights stay warm and those hookups earn their keep.
Because the sites hold rigs to 45 feet, a van isn't your only choice here; the Anaheim RV rentals page covers the larger classes.
Crystal Cove and Bolsa Chica
The Newport-coast overnight at Crystal Cove and Moro is a Costa Mesa trip; the Costa Mesa campervan page has it. The Pacific Coast Highway lot at Bolsa Chica State Beach sits closer to Long Beach, and that overnight is on the Long Beach campervan page. For anything past Orange County, the California campervan hub lays out the rest of the state.