Re: GPS
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!