Tony, Lets back up a little and start from the beginning. Your single alternator, dual inverter coach has an engine driven alternator. When no 120 volt input to the inverters is available (shore or generator power) the alternator driven by the engine provides the charging current. I believe yours has an external voltage regulator and that is what is determining the voltage to your batteries. It will generally be around 28 volts.

When the bus engine is not running, but has shore or generator power to the coach, the inverters become battery chargers. They can have voltages around 27.7 to 28.4. The range is because they can be set to charge gel cell batteries (lower voltage) or AGM batteries (higher voltage). See the link to charging below.

If your coach has been set for gel cell batteries I would guess you would see the lower voltages, but not as low as you describe.

Since you have three independent charging sources, the alternator, and the two inverters I surmise based on your readings all are working.

I can guess with confidence the alternator is working. The fact that the voltage goes up tells me that. It may be the inverters are in the float portion of the charging cycle which drops down to 13 to 13.2 (26 to 26.4) It is also possible you are set up to charge gel cell batteries.

This link will explain the charging voltages:

http://www.windsun.com/Batteries/Bat...ery%20Charging