120vac seems to be mainly an American thing. Create two 120vac legs by splitting a 240vac line and using a neutral for return (why 240vac lines have double pole switches). In places that have 220, both sides are hot.
In DC circuits it is really a loop (well AC is also just a much bigger one). Interrupt it anywhere and flow stops.
Some people think that current flow got confused from day one and is really negative to positive. Doesn't really matter except in special cases.
English cars used to be positive ground (chassis return) and it is said that Lucas put the fuses on the ground side so that the components would protect the fuses.
Bottom line is that you can open either side of a battery and it will prevent flow but the only way to eliminate the possibility of ground shorts with shore power is to use an isolation transformer. And 30A 1:1 transformers are not cheap.
What I was getting at was for a battery disconnect, it does not matter which side of the battery you pull (or the center if GC2 in series). One of
these is designed for starter current and will handle anything you want to do.