You're getting some great advice! Yes, it's a lot to learn. If you haven't yet visited Margo Armstrong's website (http://rvlifestyleexperts.com/) you're missing a great repository of excellent info, all meant to help answer the very questions you're asking.
As for the rig, remember that you'll want to balance comfort (family of 7 is a lot of people in an RV, but it's been done) with mobility -- the larger the rig, the fewer places you can get into. State parks and many federal lands have smaller, tighter spots you'll have to bypass in a bigger rig. Look harder at HOW the RV is made than what the amenities are -- you want something that will hold up MORE than you want some cute cupboards, for example :)
$500/mo to stay is do-able. Remember there are differences between "campgrounds" and "RV parks." Some places are restricted to RVers over 50 or 55. The guidebooks recommended are good ones and should help you with those.
We never RV'd before going full-time, and wish we'd followed the advice we ignored: rent one and try it out first on a familiar trip. We just plunged in, found out we knew NOTHING despite more than five years of preparation, careful rig selection, and thousands and thousands of "book-learning" pages. Even so, we can't think of anything we'd rather be doing.
Good luck!