[hiccup]

You can temporarily disconnect the wires from the switch. Be careful, there can be high voltage on the leads from the inverter/gen/shorepower. Remove the wires and temporarily (but safely) insulate the ends of the wires, see if you still trip (I would bet 10:1 you do). My selector switch also rotates past the markings, it is fine, I doubt it ever had a stop.

With that curiosity satisfied, the next step is to confirm it is a GF problem as opposed to an overcurrent problem (does it trip the generator breakers??). Once you have confirmed it's a GF problem (I'd be a taker again at 10:1), you need to systematically go through to find the problem. To me it sounds like you have multiple problems, but they may have a common root cause, tough to say. When you say the 50A pedestal trips 'right away', are you saying it trips when plugged in but all breakers on the boat (including the trunk) turned off? This would indicate a failure between the first breaker (in the trunk) and the end of the cord. The tripping you mention on 30A power sounds more typical of a boat-side GF problem, number one cause of these problems is inverter wiring (battery chargers, hot water heaters next on the list). The anecdote about previously being OK with 'upgraded' pedestals may be a red herring as the breakers may have different thresholds for fault tripping, some as low as 5mA, which is hard for an old boat to meet, etc.

Tiara should be able to provide you with the wiring diagrams (including the inverter option) for the boat, and they can go a long way towards helping, but keep in mind it is possible if not likely someone has changed things over the years.