Most privately owned RV parks we've been to allow early check in or late check out. Most without a fee but a few do charge. If a park is not full then early check in is rarely a problem. If it is full and our spot is vacant then most RV parks have allowed us to arrive early.
Late check out of 1 or 2 hours has rarely been a problem as well. As long as we ask the night before or early in the AM.
State and national parks are a bit different. Most have allowed early check in if the spot has already been cleaned. Most have not allowed late check out. We've seen workers come rake spots and clean but they never spend more than 10 minutes.