It is what it is, their campground their rules. I believe as long as your rig looks good it is probably a non-issue, most who work in RV park offices wouldn't know the difference anyway if you fudged the numbers a bit. Nothing all of sudden goes to pot when your rig exceeds 10 years.
Thankfully we only use private parks as a last resort although our rig still qualifies under the 10 year rule. If it should happen to you don't fret just move on down the road to the next park where I'm sure your money will be appreciated. Happy Trails!!!