https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html
https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html
I happened upon this website that has several calculators for backing into your battery packs required design capacity.
First figure your ac amperage that your likely to need to draw from the batteries between re-charges.
For example a residential frig when its running draws about 6 amps, which using the first calculator draws 66 amps DC, through the inverter that converts DC to AC.
Typically a res frig is only runs about 8 hours out of 24.
Figure up your total DC amps for all the gear you want to run (including your 12V stuff like lights, etc.).
Use the second calculator and the specs of your battery(s) and it will tell you how long you can draw those DC amps you added up without depleting the battery below its 50% discharge level.
If you draw the batteries down below 50% on a regular basis you drastically reduce the number of charging cycles it will accept before its capacity starts to fade.(flooded & AGM)
For example lets just use only the residential frig drawing 66 amps DC. Plug in the specs for (1) $200 Trojan 1275 12V flooded cell battery (I plug in its 20 hour and 10 hour specs). The calculator says you can run that 66 amp draw for 48 minutes and then you'll hit its 50% discharge level.
Again figure the frig only runs 33% of the time, and you can get a 2.4 hour window of time that (1) battery can support the frig. (Again without drawing it below 50%)
Plug in the specs for the deep cycle batteries typically supplied by manufactures and you'll find a drastically smaller window of time, than the Trojan's or anything of its caliber.
Hope this helped.