I don't think we can have it both ways. Campgrounds are filling up faster than ever nowadays - so that doesn't hold with the "generation isn't into camping" argument.
I think it's a matter of how people buy their RVs and their lifestyle.
First, many folks who are now in their late 40s to 60s chose not to have kids or just have one. Their RV purchasing doesn't change a whole lot between their "raising kids" period and their "empty nest" period. So they aren't as likely to trade in their rigs as frequently.
The older generations and younger generations have tended to have 2 or more kids, so that lifestyle change affects their choice of RV and will more likely result in a trade in. The younger generations haven't yet passed the raising kids stage so they stick with the one rig and plan on waiting for trading in until the kids are grown.
Then you have the simple fact that the Gen X and under are less likely to be as successful as their parents. So, that "disposable income" that the older generations had to purchase multiple RVs in sequence isn't available for the most recent retiring folks. And retirement age has become extremely variable - rather than the old standard of 65. Now you have folks retiring at any age between 40 and 80+. So, there isn't an entire generation reaching a "we're retired, let's RV" stage all at the same time and making a large impact on sales in a short time frame. Now the purchases are spread out much more.
I also think that the Baby Boomer generation is more likely to trade in their RV in shorter timeframes. The older and younger generations are more likely to stick with one for an extended amount of time. As that generation ages, those folks who were trading in for a new rig every 3-6 years are starting to drop out of the RVing world.
And perhaps the "to heck with family ties, let's go fulltime" attitude has begun to fade. The younger generation is more likely to take the entire family along or do it when the kids are young and the grandparents are still healthy.