SimchaSabre has you took care of.
As he already pointed out, the 50 amp site may also have the 30 amp outlet available like in the pic below, but if not you can use a 50-30 amp adapter that you can pick up at RV dealers, Wal-Mart in their RV section, Amazon.......and sometimes you can borrow them at the campground.
It's really a good idea to have this adapter irregardless, cause sometimes you may be in a place that has both a 50 and 30 amp outlet and the 30 amp outlet is bad or wore out. You can then use the adapter and plug into the 50 amp outlet if necessary. I have encountered that problem a few times myself.
Here is a youtube video that may help too: