WTH???
Don't "parks" have power posts with 50A 240, 30A 120. and duplex receptacles any more?
Fifty amps is serious power. A safe, long lasting "Y" connector would not be inexpensive. I think I would trust more in making my own splitter. A quad deep box. Kerney nuts. Six gauge to 12 gauge. A popout 15-amp breaker for the receptacle.
Connecting any unfused 120 volt feeder to a 50 amp breaker is nuts. Dangerous. You need to "fuse" the 120 volt circuit correctly. Then this Y gizmo, left exposed, would be hazardous in the rain.