As noted, John and Penny Thompson Park is the only County Park with a campground. It is nigh near impossible to get into it during Dec-Mar. There is also the Miami Everglades Campground www.miamicamp.com
that is a fairly decent former KOA.
Both are located in deep Southwest Miami-Dade County and I am guessing the commute to the Miami Beach Convention Center (boat show central) is at best 45 minutes at worst easily twice that.
There are a couple of more or less mobile home parks in Homestead hat have some transient sites but even farther from MB.
Might want to consider parking your Airstream at Miami Everglades (or one of the Broward County parks) and going back to the hotel you stayed at before.
Sorry I couldn't be more help,