Is a Mean Well RSP-320-15 a good choice for Top Charging?

It sounds like you have lost one leg of the 50 amp service. Inspect shore cable, circuit breaker, and neutral bus.

You might try a vise grip pliers on the stripped fastener. By pulling and turning at the same time, it may loosen. Alternatively, cut/grind the head off. Three screws will hold the cover on when you are done replacing the circuit breaker or tightening wires.