Today's young people have it much better than any generation before. A Midwestern family (the "if it plays in Peoria" definition of the average American)of four almost assuredly has two cars, several televisions, cell phones, computers, ability to eat whatever they choose and live in a home with heat, air conditioning etc. It's a long way from the "a chicken in every pot" days.
Seasonal RV parks are not in the wheelhouse of most younger people. They are much more mobile. Travel to any destination RV park during the summer and there are kids and young families everywhere. They tend to be there for a day or two and then up and on to the next destination. They are doing 15 states on their 14 day vacation. When you work 48 weeks a year, there is no need to rent an RV site for 5 months of the year.