I think a trip like you are describing would be wonderful in April, May or June when the weather is ideal in FL and the snowbirds have departed.
But most people head south in Feb to escape the cold, and maybe you are, too. As others have said, anywhere warm and waterfront is premium in S. Florida...might take a lot of phoning and/or driving around looking, which doesn't sound like the carefree trip you envision.
You did not say how big your RV is. Bigger sometimes = harder to find a site, of course. If you want full hookups, same.
The sites you do find might be, with good luck, cancellations, or possibly the leftover pickings near the highway noise, but you could certainly give it a try. The prices are going to be a bit steep. For the Keys, expect to pay $40 a night for a state park (very, very hard to get into in Feb) up to $60 - $100+ for a commercial park.
If you want to make some reservations now for February, you can still get sites at Flamingo CG in Everglades National Park, which has electric. You probably can get a dry camping site (and no pets or generator use) at Big Pine Key Fishing Lodge. You can also still get some of the state parks in the northern inland area, but it may be a bit chilly.