In the Upper Midwest spring appears in March, but not camping weather, then April comes, and spring appears more, but I like 60 degree highs with little wind, so I can set outside and be comfortable, we too end in October, so many times it is a May option, for us.
Once in a great while we can get out in April, and if the weather is nice typically we need no reservations in April, but starting in May, sad to say reservations are what camping is in today's world.
Living in Iowa, campgrounds good campgrounds are harder to find then some of our boarder states. Our state does not promote it's greatness like it should.