Originally Posted by LeonW
The batteries were near 100% before we started all this, at least according to the go power controller. So I am not so much worried about a failure to charge as I am about something actively discharging the battery while towing
The state of charge on the GoPower controller probably just indicates that the solar panel is charging the battery, not that the battery is fully charged, since the controller's estimate is based only on the voltage it sees. You said that you just took your trailer out of storage and then drove for a few hours. That would not be enough time for the battery to reach full charge.
You can get a more accurate idea of the real state of charge of the battery if you read the GoPower controller indication a few hours after dark or a few hours before dawn making sure that there is no load on the battery.
A better solution would be to get a more sophisticated battery monitor such as a Trimetric which will keep track of the energy going in and out of the battery.
Another solution would be to connect your trailer's battery up to a smart multi-stage battery charger after you bring it out of storage so that you know it is fully charged before taking off on a trip. The trailer's converter can charge the battery when you are plugged into external power but it can take several days
to get the battery up to a full charge since it charges at such a low rate.
There has been quite a bit of discussion on this topic! See Converter question