Do most state parks have a 30-foot length limit?

I would say that's not true. Some do and some may have only a few sites that long but there's no consistency about it. Older state park campgrounds tend to have shorter narrower sites but many are slowly being rebuilt and upgraded so it's just a toss up. We have a 30 foot MH and I don't think, so far, that we've found a state park that won't accommodate although I know of some that have some sites that won't work.