I can second Kevin's experience. When I towed a 77 Airstream it was "too old" for some of the parks we were in. We would pull in to the waiting area and ask if we could check in. If the age of our unit was an issue they could say something and give it the once over. It seems most of the parks that have limitations are due to their own lack of services (as mentioned) or the desire to keep a level of outward appearance. The age limit is a line in the sand way to do it. Not to offend members that have more than one RV but have you see how some of the SOB units from the 60s, and 70's look?
An Airstream will normally pass the eye pleasing test no matter the year, but then the is that pesky self containment issue.
As to the class action issue. Most parks are independenly owned and as a private business reserve the right to not accept you, or kick you out for almost any reason. State and city parks usally cannot restrict, but private parks are another matter. Just look at what is going on with Augusta National and the pressure to allow women in.

Not saying that it is right, or wrong. It just is.