IMHO, it is used to keep lower income people, and people with less than desirable social skills, out.
A 10-year old class A is generally still valuable. And Class As are generally more money to acquire. More money, which generally means a better credit score, means less domestic assaults, better behaved kids, less un-desirable behaviors, less noise, better social skills, etc.
They could probably get the same better clientele by using credit score, but that would be a nightmare to facilitate. A background check probably would not fly in the RV world. A Class A that is less than 10 years old is their way of doing a background check.
As a long time landlord, I can attest to the better income/credit scores means better tenant behaviors for a apartment (not an RV). It's not 100%, but just as not all pit bulls are bad, there are enough bad pits to keep the rest of the good ones out.