Sleep well Joe, the electrons are where they are supposed to be.......13.18V is just fine. There is circuitry in the Echo Charger that connects the battery banks, house to chassis, and limits the current that can pass to about 5 amps. The circuitry in the Echo Charger is not a perfect "switch" so you will drop about .5V in the transition through the Echo Charger. The fact that you are measuring 13.2V would mean all is well and that the house batteries are being supplied about 13.7V from the main charging/inverter system.
When you look at the guages up front, you are remotely reading what is happening at the other end of the wiring harness from the battery. Even the computer systems in these coaches are sensing voltage levels that are remote from the battery terminals in most cases. When there are questions, in order to be sure, check the health of the charging systems at the battery and you will know for sure the charge levels and the health of the charging systems.
Regards Ed