Head southwest out of Miami to the Homestead entrance and you can sleep inside Everglades National Park. Long Pine Key is the nearer of the two drive-in campgrounds and it runs November through May. Flamingo is the other, at the end of Main Park Road where the mainland gives out into Florida Bay, and it stays open all year. Book the site before you book the van. Those two are the only drive-in nights in the park, Long Pine Key is gone from June through October, and the dates still open are the dates the trip has. The van is the second booking and the easier one to move.
Neither one books on Recreation.gov. Both go through Flamingo Adventures, so an empty federal search is not the same as sold out.
You drive down here for the sawgrass and the water. Walk the Anhinga Trail at Royal Palm first thing, when the alligators and the wading birds are on the slough, then keep going south. Flamingo is 50 miles from Homestead, so fill the tank and buy groceries before you go through the gate.
Larry and Penny Thompson Park, next to Zoo Miami
Book this one when you want Miami in the daytime and a hookup at night. It is a Miami-Dade county park with 240 RV sites, each with electric, water, and sewer, and Zoo Miami is next door.
South Beach fits into the same day. Park, swim, eat, and drive back out to the campground. Nobody sleeps in a van on Ocean Drive. If the group has outgrown a van bed, size up to a Miami RV rental and you can still use these pads.
Biscayne Bay and the road south
Biscayne National Park is almost all water. Island camping is boat-in and tent-only. The van's part is the lot at Convoy Point while you are out on the water, which makes it a day and not a night.
Keep going south on US 1 and the trip becomes the Florida Keys. North and west of the city, the rest of the state is on the Florida campervan hub.