Nothing on that adapter looks like a 50 amp to me, so I don't know what they're talking about. It looks more like my 30A power cord, just a few feet long instead of 25' long.
Ok, that aside, you want to run your cords for whatever you're rated at. If you're 30A, run a 30A extension cable all the way to the Genny. If you're 50A, run a 50A cable all the way to the genny.
Then use standard RV adapters (like these: Electrical Adapters, Replacement Power Receptacles, Power Adapters & Connectors - Camping World
) to connect to the genny.
I believe there are some generators out there that can even supply directly to a 30A so you don't need to use an adapter, but I don't know if they're in the size you're looking for.