My Garmin Street Pilot 2720 shows RV Parks under the lodging poi's, but is not all inclusive of ALL the parks in a given area. I got the Garmin from Wal-Mart Web site for $449. At the time (May 07) some of the other Web sites were selling it for $899. It comes with a really neat dash board bean bag mount that is portable.
Like DL says they don't always have all the parks that are available to you. (Every time I go to a park that isn't in my Garmin 7200, I ask them to contact Garmin with their coordinates.)
Probably any brand of GPS is OK. The differences are in how you get to interact with the data it provides. Try them all out and see that you can get around in the menus and understand routing and POIs. One of them will click with you and how you think.
You won't be able to just tell your GPS, "Go find me a place to stay tonight." We need to use a combination of maps, directories (Trailer Life, Woodall's, etc.), and GPS to get around. Oh, and a cell phone!