From practical tests I've done, my inverters cuts out at about 50% of the capacity of my batteries. For example if I have a 63 amp battery, the inverter shuts off after I've used about 30 amps of it.
That is why I figured 2X 102 amp batteries @ 50% is 102A. Which is easily more than 75Amps needed over 3 hours. It does not make sense to me that 6 batteries could not pass 50 minutes. I am not saying it did not happen 
Just that it should not really happen.
You cannot run down lead-acid, whether flooded or gel, to 50% depth of discharge. Run a 12 volt battery a few times under 11 volts, and the recovery is compromised. I have had customers who did this and now they find when they have no AC power after dark, their inverters or UPS batteries let them down. Instead of getting a couple of hours, they get 20 mins or less
On the other hand, if a flooded or gel battery is properly managed, it will give 6 to 10 years of life. I've seen the brand "Power Sonic", made in the USA, last for 15 years
If you want a fit and forget battery, it is a lithium-ion
Expensive to begin with, and it needs a well set up battery management system, but a) it will last, b) supplies all the power you need and c) is a fraction of the size and weight of a lead-acid battery
If lithium ion is good enough for Tesla and most other electric cars, its good enough for me
