I don't think any of the obvious routes would be terrible, though I'm not super familiar with them by any stretch.
I would suggest going in at the Perry Entrance, which is closest to the campground. You'll avoid tooling around the park for several miles. Most of the popular attractions are at the other end of the park (nearer the Castile entrance).
As a first guess, I would probably take I-79 to I-86 to NY 19 to NY 39. The state routes are two-lane roads through towns etc., but at least at a quick glance don't appear too terribly bad to travel on with a motorhome.
