Looking at new trailers, they appear to be priced about the same as what they were when I bought mine 6.5 years ago. Cars have gone up, but not more than they ever do. Gas has gone up (although not for a couple years now). Some campground fees have gone up - this varies quite a lot by state/region. Around here, they're not that much more.
In the meantime, airfare has also gone up a fair amount, as has the price of staying in a hotel.
Seems like a wash to me. If I'm going to go on vacation, I'm going to travel somehow and stay somewhere. The options here have all gone up. Camping is about the same value it has been, relative to the other options.
I think part of the issue here is that while these costs have gone up, incomes among the middle class by and large haven't. Incomes in the top 1% (and even more so in the top .1%) have gone up a lot, but that's not most of us.