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

As others have said, you won't have a problem at all in your home state and most others. One exception is California. Many state parks have a limit close to yours for several reasons like space, terrain, and high demand. When we head out of our home state of California to RV, we go from being the largest rig in a state park to being by far the smallest rig in a state park at only 25ft in length.