[hiccup]

I think you have a couple things going on.  

Based on your picture w/the Xantrex controller, it appears that you have the inverter option (which is really an inverter/charger combo).  In this case the C-Charger only charges the engine batteries and the inverter/charger charges the house batteries.  The Xantrex controller you're looking at is telling you the charging voltage for the HOUSE batteries only but not telling you at all what the engine charger is doing.

If the batteries are at 6-8 volts with the charger ON (C-Charger in this case), one of two things is likely happening.  Either the wiring between the charger and batteries is not electrically continuous or the charger is malfunctioning.  The most likely reasons for lack of electrical continuity would be the DC breaker switch under the stairs "Engine Charger" is OFF or the fuse in the C-Charger is blown.  Did you measure the charging voltage at the C-Charger with a multimeter?  What is the ammeter on the C-Charger reading?

As an aside, the inverter/charger likely can be reprogrammed to be compatible w/AGM batteries.  Furthermore, while Gel batteries are not compatible with AGM charging profiles (Gel batteries are very voltage sensitive), AGM batteries may very well be compatible w/Gel charging profiles--the key is to find out what the charging profile of the C-Charger is (ask Charles) and then compare that to the specs of the actual AGM batteries you're looking at.