Robin,
That sounds about right. I too am left with 12.3 volts after and evening of fun when dry camping including sat dish, 2 tv's, Samsung Refer, Bose system and microwave.
Here are some considerations. Inverters normally require 10 times more 12 volt energy to make it into 120 volts ac house power. Example would be a microwave which is 1500 watts. 1500 divided by 120 volts equals 12.5 amps of A/C power. Multiply 12.5 X 10 = 125 amps of 12 volt power. Now keep in mind you only run a microwave for a few minutes at a time so don't let that alarm you. My Samsung RF-18 requires 1.2 amps so it pulls 12 amps of battery power when it cycles 3 times an hour. You are correct not to discharge your batteries beyond 50% of there capacity. 11.2 volts equals a "dead" battery. 12.7 to 12.8 equals a fully charged battery. 12.0 volts is 50% discharged. Hope this helps, Fred