The physical plug is different for 50 and 30 amp connections, so you need an adapter. They're usually called "dog bone" adapters because they're pretty short with big plugs on both ends. Here's the one I have:
https://www.amazon.com/Camco-55175-PowerGrip-Dogbone-Electrical/dp/B000BUTDCA/
https://www.amazon.com/Camco-55175-PowerGrip-Dogbone-Electrical/dp/B000BUTDCA/