Generally, this restriction is pretty flexible. Frequently, a park with such a rule will ask for a photo of your "older" coach and if it is in decent condition, will waive the rule. I have seen coaches older than 10 years that looked like they had just been driven off the showroom floor, and newer ones that didn't.
The 10-year rule is used primarily to screen out old RVs in bad condition that leak oil and other fluids, suffer mechanical breakdowns and otherwise become a problem for park owners. We can debate the pros and cons of such rules 'til the cows come home, but they are what they are and, park owners are legally entitled to make them.
Take a few photos of your coach (inside and out) and have them available in digital format if a park asked for them. You should have no problems.
TJ