Rather than disconnect your cruising
batteries, and run directly off a battery charger/power supply, I’d install a cheap “throw-away” buffer battery. Remember to maintain your cruising batteries, while they are disconnected.
Of course, you won’t be totally independent of 120VAC - your charger requires
shore power. The
shore power supply should be a GFCI protected outlet, with both primary (120V) & secondary (12V)
fuses installed at the charger.