Check with the park. No one on the forum knows why they chose that rule and how they enforce it. It’s a county park, meaning the government is involved. For all we know there was a serious accident involving a home built rig. Maybe a fatal fire or electrical issue. The county attorney defending the county does some research and decides the RVIA sticker would provide them a layer of protection in the future so it is mandated. The employees aren’t empowered to make any exceptions so no sticker, no site end of story.
As for all the posts saying they have never been questioned, they probably never have had a lot of things happen to them but have happened to others. That’s why people have insurance, door locks, bear spray etc. Just because it didn’t happen to Fred doesn’t make Barney immune.