house batteries

So confused. I need new house batteries. I have no idea how big I need them to be. I would like big enough to stay at anchor for 2 or 3 days. Online says to measure all the amp draw, but I have no idea how to do that either. My Sea Ray 275 Sundanacer has a fridge, vaculush, TV, radio, lights...

95 300DA Sundancer with 3 batteries running bow to aft on the starboard side. Which battery is house battery? I assume: Rear bat = Port Middle bat = House Front bat = Starboard Can a few people give some feedback please. Thx.

We almost didn't take this vessel over initially due to the bypasses and work arounds but everything works, SO ... Last visit I went through the batteries, cleaning and servicing them and tracing the wires noting their condition and whether or not they were run correctly or rerouted or bypassed...