question

I presume that SOC and TTG just show three --- dashes?
If so, this would indicate that the BMV lost power at some point. In addition, the low Voltage readings indicate a poor connection at some point.

I'd suggest starting with the red wire that goes between the shunt and the battery. Open up the inline fuse holder, sometimes corrosion can set up in there.
After that check the cable that runs between the shunt and the BMV it's self.
Lastly, check the negative battery cables attached to the shunt, although it's unlikely for there to be such problems here otherwise you'd notice more disturbing issues.

Thanks, WKirby. That is helpful. You're correct: SOC and TTG just showed 3 dashes.

I checked the red power wire and fuse. All OK.

I didn't really check the cable to the BMV, the BMV is on and I'm getting some readings.

I did do a manual synch on the BMV by holding + and - buttons. After doing so the TTG and SOC now have readings.

With the charger off, the BMV now reads 12.7 V, and when I put my multimeter on 1 of the batteries in the house back, it' reading 12.6x.

Turning the charger on the BMV reads 13.x (it's increasing, I think it tops out around 13.65). I'm wondering if my batteries are toast--they are probably 5+ years old AGMs installed by the PO.

The BMV600 can switch off at low (12V battery) voltages, if you had a very low battery event it will reset and forget SOC.

The BMV700 can go a bit lower before resetting.