It will be hard to find too many places for $900/month in SW Florida and with a reasonable drive to the beach. The farther south you go - the higher the rates. We like the Sarasota and Fort Myers - there are several smaller parks in both areas that may meet your needs. I can't give any names, since we always stay at Sun-N-Fun in Sarasota or Cypress Woods in Fort Myers - they are both higher than $900 in the winter season.
Wayne