Almost always, the 50amp site has both a 50amp plug-in AND
a 30amp plug-in. Many times, they'll even have a pair of 15/20 amp plug-ins.
In the unlikely event that they don't, RV stores carry a 30amp (female) to 50amp (male) adapter that makes it so that you can plug your 30amp cord into the 50amp outlet.
It might be worthwhile calling the campground to see if they have both of if you need to pickup the adapter ahead of time.