Letchworth is a really nice park...quite large. So, you will have plenty of miles to ride bikes or hike. And by being there in June, most NY schools are still in session unil the last week of June, so it shouldn't be crowded except on the weekends.
It's been a couple of years since we've been there, but they were upgrading a lot of their sites which would give you great hookups for a class C. Not sure if they have any sites with sewer hook up, but the bathouses in the NY state parks are usually fairly decent. Water access is usally near the campsites so it isn't too difficult to keep your water tank filled. I'm not sure if they have any campsites with water.
The Genesee River gorge is really nice, and there are a couple of waterfalls. If you have a few bucks to spend, the hot air balloon rides that launch from the park are incredible. My wife & I went up back in 2005 as I received tickets as a retirement gift. The experience was incredible.
If you like amusement parks, Darien Lake Park is not far away.
Hope you have a great trip!
2012 Newmar Bay Star 3302
Blue Ox Avail
BrakeBuddy Advantage
2015 Malibu
"Get busy living, or get busy dying."
Andy Dufresne, The Shawshank Redemption