Try checking hot-gnd voltage for each phase with a meter at the pedestal, then at each end of the splitter, then at the far end of each cable, then at the back of each shore power inlet to see if you can find where the voltage drops off.
If that doesn't turn it up, disconnect the cables at the boat end and check neutral-gnd connectivity at each far end of the cable, and if it isn't very near 0-ohms, wqlk back step by step, connection by connection to the pedestal to see where neutral and ground show open or high resistance. Be careful not to short either to a hot prong while testing. Since the pedestal breaker likely opens neutral, you'll need to be hot while testing.
Alternatively, check hot to neutral and hot to ground voltages at the pedestal for each phase as well as neutral to ground. Then, take the pedestal end of the cable to the boat and bring it to each of the inlet side cables. Check for 0-to-very-low resistance for corresponding pins and open pin to pin for all other pairs.
Don't discount the possibility that the pedestal lost its ground.
If thatbdoesnt turn up the problem, connect back to shore and boat, turn on at the pedestal, turn off all breakers, and check for 0-to-low resistance neutral-bus to ground and line voltage for each phase to neutral and ground. If you have an inverter neutral bus and a regular one, test from the regular one.
The goal is basically to see where ground stops being a ground.