In ten years of RVing I've encountered maybe a half dozen parks where a site with 50 amp service had a different daily rate from a site with 30 amp. In these cases, I'll ask for the 30 amp sits.
Otherwise, where 50 amp is available, the electrical service box usually includes 30 amp and 15 amp or 20 amp outlets as well.
Only once have I been put in a site that had only a 50 amp and 15 or 20 amp connectors, no RV 30 amp. However, I've been put in sites where the 30 amp socket had corrosion problems, and the 50 amp socket was in better shape. I find it helps to have that 50->30 dogbone adapter with me, to deal with bad outlets and the occasional 50-only site.
It us much more likely that you'll encounter sites with 30 combined with a 15 or 20 duplex, particularly in older RV parks and many public campgrounds.