"Though some of the responses suggest otherwise, Two batteries in good shape will/should run your MW without a hitch...I use our MW regularly from two 6v batts..."
Yes, but the two need to be over 75% SOC to keep the loaded voltage above 11v. With four 6s, you can run the MW at 50% SOC and stay above the 11v inverter alarm.
On solar, you are usually always above 75% SOC, but not when you are doing standard 50-90s no solar.
OTOH we have posted data from owners that shows you can get away with a smaller bank and stay above 11v if you have AGMs instead of Wets. AGMs are more like Wet 12s but better, in that regard.
Lots of situational stuff here, so hard to make general rules of thumb.