William,
A fully charged 6 volt battery will read in the 6.3 to 6.4 range after setting for an hour or so after being removed from a charger. However, the 6 volt batteries used in our coaches are wired in a series of 2 batteries to deliver 12.7 volts when charged.
If you are having low battery voltage issues when plugged into 50 amp shore power, you need to have you inverter serviced or replaced.
Gerald