Feel your shore power cable, and end connectors for heat, while you have a load connected.
Open all breakers so there is no current
through the board. Check voltage at your new shore power breaker input and output. Do the same with everything on the boat
turned on... lights, fans, reefer, radios, everything. Test and reset the shore power breaker and do all that testing again. This may well narrow down the search for the fault.
Double check your connections and make sure you have the 120v hot leg going where it is supposed to go, and the 120v neutral leg going where it is supposed to go. And the ground going where it is supposed to go. Is there a chance that you switched neutral with ground?
Your voltmeter is set to AC Volts, right?
Does stuff seem to run okay? Plug
in a blender or something for just a second or two and see what happens.
If you have a high resistance somewhere, it will cause a voltage drop, and the high resistance will dissapate heat. You don't want that. More current = more heat. Too much heat and insulation
melts or burns and you get a short. Boooo, shorts.
Did you also by chance install an isolation transformer?