I haven't had any real problems finding campsites for a 32' class C in New England. The selection in some places can be somewhat limited, but with a little flexibility and/or advance planning even there it's usually not a big problem. I think a 25' unit would be even less of a problem.
Of course, there are all different sizes and types of campsites, and it goes without saying that a smaller RV will always fit into more places than a larger one. For that matter, there are quite a few sites in the Vermont state parks (and elsewhere) that only have walk-in access and so are only usable by tenters. Everything has its tradeoffs; but I don't think you'd find a 25' RV is particularly limiting.
Sometimes the size limits claimed at some parks are quite conservative, though not always. For example, in the picture below (at Watkins Glen), I was technically two feet longer than the stated maximum for the site...and, as you can see, actually only took up maybe half the space available. In this case, at least, the access roads were not any obvious limitation either.