QUOTE:
"My question regards sitting at the dock. Shore power is plugged in, the battery charger is charging and the new fridge would be drawing power from the (grp. 31 DC)house battery. What happens with that? I assume that the battery charger senses that the battery needs to be charged so kicks up from maintain mode to a charge mode? What happens when it runs 24/7 during the season? Does it boil the battery or? Normally the charger comes on, does a bulk charge, steps down to a lesser charge level and finally steps down to a maintenance charge level."

The answer is "it depends"..

I see a lot of batteries live very short lives dockside as the result of crappy so called "smart chargers"... Some chargers are very sensitive to float/absorption triggers and others are less so. Some simply revert to absorption if terminal voltage dips to 12.6V, some have a timer at low voltage and some just use a percentage of the chargers power supply as the trigger to start another absorption cycle. The safest solution is to use a charger that can be set on "power supply" mode (basically just constant float) or one that features a custom program where you set absorption & float voltages to roughly the same level thus constantly floating at the dock between uses.

It is essentially the charger that is supplying the power to the fridge the battery is just in the circuit so you really want to ensure the charger is not triggering an absorption cycle every time the fridge kicks on...