I've never been able to determine whether there is any place in the US where the nominal is actually truly 220, or whether the nominal is really 240 everywhere here and the 220 number is just a relic of the past. In any case, in the US, consider the two to be 100% equivalent. It's like toe-MAY-toe vs toe-MAH-toe: either way, it's a juicy red thing thrown in cartoons at poor performers on stage.
Anyway, what bansil said is right. The 220/240 is "split phase" which means two 120 V supplies. The voltage from either hot to the neutral is 120, and between the hots is 240. If you only want 120 then just pick one of the hot legs and ignore the other one.
There's a picture of a typical 50A 240V campground pedestal there
. (that site wouldn't let me deep link to their image directly) You could connect to just the ground, neutral, and one hot blade of the 50A receptacle or, more conveniently, use the TT-30 or 5-15 plugs instead. You'd have 120 either way. It isn't that most parks "only" have 240V, but rather they advertise that because it's a selling point -- they'll also have the traditional 120V sockets available too.