That is not a 50 amp 4-wire receptical nor a 30 amp RV receptical. It looks like an older three wire 220 VAC receptical, which would have 220 VAC between the two vertical slots and a ground wire.
What ever you do DO NOT PLUG YOUR RV INTO THIS RECEPTACLE!!!!!
Great damage will be done, costing you thousands of dollars.
Rick & Debbie; Brandy & Schnoodle Dexter & Fritz R.I.P. the Doxie "Kids"
2015 Jayco Pinnacle 36RSQS 5'er
2013 Chevy Silverado 3500HD LTZ, 6.6L Diesel Dually; B&W Goose Neck and Companion 5'er hitch