25 footer worked for me, I have a small camper. You might buy 2 25 footers and a barrel connector so you can join them up, oughta be able to park anywhere on the pad.
It should be long enough to get from your cable hookup on your RV to the post with the cable box thingy. Sorry, couldn't resist.
From my experience I wouldn't think you wouldn't need any more than a 25' cable, but just to play it safe I would do like the previous poster mentioned and bring 50'.
My cable jacks are at the far back end of my trailer. Campgrounds seem to put their connections anywhere. I keep two 50ft cables on a spool and 2 shorter ones. Probably 10ft and 25ft. That way I can make it reach no matter what. I also have the satellite dish and sometimes I need over 100ft to get clear of trees.