[hiccup]

I'm assuming the boat has dual 30A shore inlets?

Switching the shore cables was a good idea.  The path from shore cable to panel is:
-shore receptacle
-wires
-breaker in cockpit
-wires
-breaker on panel
-wires
-panel

It would stand to reason the problem is with one of those things or the connections between them.  At almost 20 years old it is a good idea to visually inspect all of the connections by removing them (receptacle, breaker, back of panel), and if you're comfortable enough around high voltage, take voltage readings at each connection point.  If you do this the problem will reveal itself.