Advance reservations are not a bad idea, particularly for longer stretches. During the week most places it's not a big problem, but some do fill up on weekends, and some parks (particularly some state parks) do not have many sites that can reasonably accommodate larger RVs.
It's not really necessary to make reservations for the whole summer right now. The big holiday weekends might be exceptions; in many cases, the sooner the better for them, especially if you have a specific place you want to be.
If you're flexible about where you stay (and when you might need to move on), your need for reservations far in advance goes down considerably. It's a very rare weekend that there are no places available.
I like the Vermont state parks, though I may not be entirely unbiased in the matter, but do be aware that none of them offer any hookups. For hiking, the parks in and around the Groton state forest have a nice set of interconnected hiking trails that are not too strenuous.
In New York, there are some exceptionally good state parks in the Finger Lakes region. Probably most famous, and justly so, are Letchworth State Park and Watkins Glen. The former is quite a large park and could easily occupy one for several days.