Yes, the plugs are big... but they need to be to stay cool and still carry that much current. Perhaps technology will give us cooling, microwaves, etc that use less current, someday and the current-carrying hardware can be smaller.
The easiest way to describe the 2 different plugs is to compare them to the old style dryer plug and the new style. The old has 3 prongs... 120V - 30A hot... neutral... and ground. But for the old dryer, it was 2 hot prongs, and a neutral, no ground. The new has 4 prongs... 120V - 30A hot... neutral... 120V - 30A hot... and ground. The adaptor just drops a hot leg and you're still hooked up as normal with a 30A trailer. The second hot is usually just for a second A/C, fireplace, or both. That's the case for TTs and probably 5ers. Larger 5ers may share more of the load for other things on the second leg. I have no experience with motor homes, so I don't know if they're the same, but suspect they are.
Is something is wired incorrectly in the coach, with the 50A plugged in, you could end up with 240V across something designed for 120...
But this will not happen to a coach using a 30A plug and a 50A adaptor... since one leg is dropped.