So many configurations and variables. On my coach the alternator will charge the house batteries. So in theory I could use a really small battery. Now, that said, if I want to park over night and have the inverter run the residential refer I need a bigger bank. Minimum 4 batts are suggest. But it comes down to math. How many watts does your refer take to run? My Samsung uses 132 watts (120volts*1.1amps). Convert watts to DC Amps and you get 11. 11*12 (# hours you want to overnight on the inverter) and you get 132 amp hours. That is just for the residential refer. That does not include the water pump, the tv, the lights, and anything else that will suck juice out of the batteries. Never mind the parasitc load on all RVs like the EMS system , the LP detector, the CO detector, etc.
A 2 x 6volt battery bank has about 110 usable amp hours in it. See the problem now?
So, the advice the person gave you is sound but for the wrong reasons.
I don't make it much on here anymore so will not see your reply to my comment. Drop me a direct message and when I come back on I will see it.