It has been my experience that they do 'not' list the 10 year rule on websites etc.
The rule is something that the CG's use "at their discretion".
And for the very reason stated earlier. If the rig looks and is a junk they use the rule to turn you away. If your 16 year old RV looks clean and kept up when you pull in.....you most likely will never know they even had a 10 year old rule as they sign you in and issue you your site!
My first RV over time became over 10 years old! It was polished and clean at all times. I was NEVER turned away from any CG's. And that was RV'ing Full-Time where I will use hundreds more CG's than the week end RV'er! :C
That said I've seen some resort areas where when you call to reserve they ask how old your RV is. If it is older than 10 years they will simply tell you to come on down and if your RV is presentable they will allow you to stay.
I for one like this practice. I do NOT want to pull off the road for a couple days of relaxation and be parked next to run down dilapidated RV's that look like they should be in a junk yard. And plse spare me the oh the humanity that not everyone has money for a new RV. It doesn't take any money to use your elbow grease to clean an RV. And THAT is what the CG rule is for! :B