OP Retest and be clear about a "plug" and isolate the problem. Check the pedestal 50A plug: 120V from each hot to ground or neutral, 240V hot to hot and 0V neutral to ground. Repeat for the RV end of the cord. Next check inside the RV panel for the same voltages. Also use a 3 light household tester on the inside 20A plugs to be sure the wiring is correct and do this for the 30A adapter. Plus check the 50A female plug on the adapter as above except hot to hot should be 0V. What are the results?
Do not check voltages with a voltmeter if you are not sure about all safety practices.