Whether it's 30A or 50A, you can use a site with either type of connection. For a 30A site and a 50A trailer you definitely need an adapter. Usually 50A sites have a 30A socket as well, but I'm sure there are occasional exceptions and sometimes the 30A socket may not be in good shape, so an adapter in that case is not useless either. It's also handy to have a 15A to 30A adapter in either case should you only have a 15A socket available (i.e. a normal electric outlet).
Of course, with 30A electric service, you have to be careful about how much power you use at any one time: generally one air conditioner and one more high-power appliance is about it at any time, with high power appliances being things like a microwave, electric water heater, hair dryer, space heater, Keurig, etc. With 15A service, you're naturally even more limited.
