If that is from the chassis, the voltage you would read should rise as the battery charges. Basically the alternator pushes current into the positive side of the battery to fill it and as it charged that battery voltage rises and it should reach 14.4V as the battery is fully charged. The voltage regulator then drops the output from the alternator down to something around 13.6 - 13.8V and maintain that. When things are working as they should, there is a solenoid that connects the output of the alternator to the coach battery(ies) to charge them also. If your coach still has the original converter, it should supply about 13V to 13.5V but no more. As with the chassis battery, the voltage would depend on how nearly charged the batteries.
A typical wet cell, 12V battery is actually at about 13.6V when fully charged and slowly drops to 10.5V under normal conditions, but if kept in service without charging it could fall much lower and a damaged battery can read far lower.
You should be able to find a battery cable that goes from the coach battery to the cross-connect solenoid that is probably located somewhere near the chassis battery, usually on the firewall for the engine. It would also have another battery cable from it to some common connection with the chassis battery and a small wire that is the control power to close the contacts when appropriate.
I hope that this is some help.