In some RV parks you may find "big rig" sites that do not have the 30-amp connector in the box. I've not yet encountered this in a public campground, but I mostly go to old campgrounds that do not offer 50 amp service at all.
You will not usually be assigned to a 50-amp only site if you need 30-amps, because park owners like to save those for folks willing to pay extra for the 50-amp service. But it can be all that is available when you arrive.
Park stores often have an adapter to sell, sometimes a loaner or rental, but I preferred to buy one of my own, of the quality I wanted. I've used it twice to work around a bad 30-amp connection, and loaned it as an extension to a fellow RV club member who found himself about a foot short on his 30-amp cord.