We avoid holiday weekends during the summer. We can check-in on Sunday and be gone on Friday. I heard that the popular parks in CA are getting quite crowded, but that has more to do with over-population than any demographic buying more RVs.
Florida, south of I-4 in the winter is getting difficult to get a reasonably price spot for any length of time. This is mostly baby boomer snowbirds. If you want to stay north of I-4 or spend $100 a night, there are plenty of spots.
I wouldn't worry too much about millennials. The few I know don't have the patience to stick with anything very long. The RV fad will fade shortly after the tiny house fad and all the other HGTV touted activities. Most are not willing to make a lifestyle change.