Like most things in life you will find certain groups who wish to exclude others, class A only RV parks are just one more example of this, since buying my current coach in 2016 and spending on average 75 nights per year in it on the road until this year I have happened upon one class A only RV park in searching online for potential places to stay on a trip. In this case, even though I have a class A motorhome, I would have been excluded from this particularly class A only RV resort, as my class A was too small, being only a 28 ft class A, and with this particular place not only did one have to own a Class A, but it had to be over 34 feet long.
You are far more likely to run into RV parks with max RV age limits than class A only parks, these typically have a max age limit of 10 to 20 years, and here again I have only ran into it once, last October in Arizona. We spent 1 night at a park with a 20 year max age limit with "no exceptions" clearly stated on the web site, and when we were there our 18 year old coach was one of the nicest looking RV's on the property, the place was surprising low end, with lots of low end 15 or so year old worn out travel trailers. But again, there park their rules.
As to the rest, finding places that accept Class B, Class C's, travel trailers, etc. of any age (if they are in good cosmetic condition), is rarely a problem, particularly for overnight or short term stays. Amenity levels vary, public operated campgrounds in state parks, COE parks, will often have water and electric hookups only with communal dump station near the entrance, each site will tend to have a picnic table, often on a concrete patio area, and have 40-60 feet between spaces. Commercial RV parks regardless of their luxury level tends to be more tightly packed, tough nicer ones may have nearly as much spacing between sites as state parks, etc. others will have you squeezed in like sardines with your neighbors 10 feet or less away.
As to sites being booked up a year out, a lot has to do with what part of the country you are in, when you want to travel and how flexible you are. Living in Louisiana in the southern US, and traveling mostly in the shoulder seasons, I have yet to see it, having a relatively small coach helps also. Since buying our current coach in 2016 our travels have ranged from Florida to Yellowstone, to Arizona and north as far as Missouri , only once have I had difficulty getting reservations about 9 months in advance, and that was for yellowstone and the big solar eclipse in the summer of 2017. Even this worked out, it just required a bit of rearranging of the route making a clockwise loop through the Grand Tetons NP, Yellowstone, etc. instead of the originally planned counter clockwise loop, and spending about 3 days on the phone trying to find a good place to see the eclipse, we ended up at a spectacular site at a hunting camp with 14 RV hookups, just a few hundred yards off the centerline of the eclipse about 15 miles down a secondary highway outside Douglas, WY. Of course for the event they had about 80 tent campers. Still a nice big open valley, and no clouds at the right time.
p.s. See attached photo of one of the nicer commercial campgrounds where we have stayed in Dubois WY from that trip, right on the water with a small river passing by, and nearest neighbor about 50 feet away. Also don't let size scare you into buying something too small for you to be comfortable in, there is a big difference in living space between a 24 ft even just a 26 or 28 ft coach.