Power problems & SOC on Steca PR2020

Hi
We've been on the road for almost 3 weeks now & have had a lot of power problems. The van has a 105ah deep cycle battery which showed 100% SOC before we left. We also had a dual battery system installed in the car with the 2nd battery identical to the van battery. The van is a Golf Outback with a single 80 watt solar panel, and a smart charger for 240v mode.
Because we have an inverter & a couple of 12 volt sockets not hooked up to the regulator we were running the Steca in voltage mode. We haven't used either inverter or additional sockets at all since leaving home. After 3 days in the bush we had no power. I swapped the 2nd car battery & van battery, charged them both fully & bought an additional 97ah deep cycle battery. I also disconnected the inverter & 12v sockets & swapped the Steca to SOC which showed 100%.
Within 2 days the SOC was showing 1%, so I installed the new battery which only showed 62%. The next evening the Steca was flashing, with a voltage of 10.7 but still SOC at 62%. I swapped batteries again, SOC now 15% & then back to the new one again. SOC showed 46% but I charged it to float, still showing 46% with 13.7v. I also disconnected the 2nd battery so no charge is going to it. Having done that the voltage has been pretty stable for 3 days but SOC hasn't moved. Checking voltages with multimeter confirms the voltage showing on the Steca.
What is happening? I don't know whether I have a battery problem, regulator problem or no real problem. Is it just coincidence that voltage has stabilised since disconnecting 2nd battery or does the car need some other controller to make sure the van gets first feed?
I'm thinking maybe an auto-electrician may be the answer but any other ideas or suggestions would be appreciated.

Thanks

David