6 volt battery issue question about load

rockwind1 wrote:
a real load on them (like pulling 7 amps thru the xantrax 1800 inverter),

Others assumed (and perhaps correctly) that you mean 7 amps @ 120v is coming OUT of the inverter. That would qualify as a 'real' load and require 70+ from the batts. Yes, that will draw them down.

rockwind1 wrote:
from what I understand, going down to 12.3 volts, you have essentially used 40-50% of the battery capacity?

Not in the situation you describe. That figure represents resting voltage, which would occur a few minutes after your 70 amp draw is completed. 4 Deep-cycle batteries struggle to produce that much amperage, compared to a car battery which probably would not.

Here's a recent thread on what you're doing:
Mix AGM and acid 2013