Have you been checking your battery/batteries occasionally, for water level, and for voltage. Unless neglected, your battery should last 5 years or more. A battery that has been used to 50% or less, a number of times will be short lived.
I would check voltage with a meter, hoping to see 12.6 or more, when not plugged into shore power. While the panel readings are helpful, it does not have the accuracy of a digital volt meter.
It could be a problem with your converter/charger, but not likely on a trailer that new.