A couple of observations.
The first one would be that two 6V 125 AH batteries with a 3000 watt inverter would be undersized. Not a huge issue depending on how much you use.
The two solar controllers rarely work in sync with each other unless there is and established master-slave designation. Again, that is not a huge problem, but you may not be getting much benefit out of having the two systems as you think.
You have been undercharging your batteries, 14 - 14.5 is usually the bulk/absorption range of your battery chargers the fact that you did not see them leave 14 - 14.5 would mean they never made it to float.
Now that you have more sun, you said your batteries never got above 13.5. 13.5 volts would be in a reasonable float voltage range, but you should have seen the still transition from to the absorption phase 14.2 - 14.5 then drop down to 13.2 -13.5.
Although the chargers may be temperature compensated ( each charge source would need a temperature probe on battery) the temperature compensation is higher voltage the colder it gets.
Typical charge routines start in the bulk phase, this will put out the max current that is available for your charger. The voltage will generally creep up in voltage as it becomes charged. At some point 80 to 90% charged it will hit the absorption voltage threshold, at this point it will move into the absorption phase. This phase will hold the voltage at absorption and start bringing the current down slowly. This is the longest phase of the charge cycle, and one that your solar controllers need enough current to maintain that charge. It is not unusual to see them drop back to mppt mode as the sun goes in and out.
So it sounds like you have been undercharging the batteries, this is more common than over charging them. Standard agm batteries do like to be brought to full charge as soon as possible, repetitive partial charges do take life out of the battery at a faster rate. It also does not bounce back when being fully charged.
The biggest solar problem we have in our area in the winter (besides lack of sun) is the angle. If your panels are flat on the roof their out put is diminished. I have set of auxiliary panels that I can deploy angled to the sun and can match the output of the two flat panels with have available power in the aux panel.
I live in the Portland area, if you wanted to stop by to discuss. just PM
2004 E350 EB V10 E/PH 2WD