Every loose connection on my twin 30 amp shore cords has been on the DOCK side, not on the boat.
Last winter, the dockside cord came loose, so the battery charger quit, refrigerator quit, and some things rotted and only now is it smelling ok again. It sped up 3 batteries replacements, which were only 3 years old. My old dumb Rairitan charger then cooked all 4 batteries, dried out the water, so I got a new IP22 Victron 30 amp 3 bank charger. Instead of then replacing the gen start battery (#4), I ran big wires to the buss bar for the start battery from generator and it starts off that now along with 2 main engines.
I can rotate house switch to 'both, 1+2 position', and the house battery joins with the start battery, just in case of emergencies. My number 2 position on the switch was not used, now it has a wire running to the start battery buss bar.
What i noticed, is I may not have fully twist locked the cord into the dock socket. What I also do now is tie it in with a 1/4 rope to the dock power pole. It can no longer come off.