Checking voltage on connected 6 volt batteries

Yes, you can test voltage on a single battery when they are hooked in series. Just remember that voltage can vary significantly depending upon whether or not there is any load. If you are testing house batteries, a good idea is to be running the microwave or another high amperage appliance when testing. Also remember to have the rig unplugged and the generator not running since that would activate the battery charging functions of the converter.