I would give it a shot in a small B van but bring along tenting gear. You will be very visible in that site, and the camp host might deem your van to be longer than the official 15 feet if it seems too stick out into the roadway at all.
If so, you can just set up your tent, unload your stuff, park down at the far end and walk back(the map does show 60 sites and a loop at the end of the road for extra car and boat parking). It's not miles away. People stroll from one end of that campground to the other for morning walks.
The site description fine print does actually read as follows: "Tent or Van only; Pop-up's will not fit." Interesting that "van" is mentioned.
But be aware - those first few tent sites are pretty exposed - they really are right by the road with minimal driveway length and not much privacy from foliage. So you are smart to keep checking for cancellations in bigger sites.
I almost booked a tent site by mistake for Bahia Honda one time in my excitement to actually find any availability. So I sympathize.
