I think your idea is a good one. It wouldn't be any different than providing WiFi access in hotels like Wayport and others are doing. In fact, you'd probably have a better revenue stream since the RVers stay for up to two weeks at a time. (Longer with park switching for the 'week out' requirements.)
Several places offer ISP "in a box" solutions (you have to get the high speed line yourself) and they provide the billing service and split the revenues with you.
A couple that come to mind are Boingo
http://www.boingo.com/hso/wisp.html and Fatport
http://www.fatport.com
Let me know if you develop this further. I live in AZ (Winter RV heaven) and there are quite a few parks near me.
You might also look for parks that offer a place nearby for the 'week out' requirement that many membership have -- that way you can cover them in both parks!
Good Luck!