Hi barrie elliot,
Your request will be difficult to close on. At this late date, consider getting a CG directory or Google "florida campgrounds" and start calling around. Unless the economy will prevent RVers from wintering in Florida (and this is not projected to happen) $400/month may be difficult.
This is not in the Tampa area, but not far away. Consider trying here
. It is more than was mentioned in your OP, but pretty close.