If your trailer has been sitting for "a few months" with no power input your batteries were probably dead. You state you plugged it in and is now reading 13.x volts so apparently you lucked out and they came back to life. Just the normal parasitic drains in the trailer would draw over 5 amps a day from your batteries. You don't say which batteries you have but they also have an additional monthly self discharge rate of up to 12%. Every time the batteries get depleted they loose some ability to recover. Several depletions will ruin them so you should never leave the trailer without a power source be it plugged in or out in the sun. If you have to store it covered and without access to 110VAC you should unhook the batteries to prevent them from draining so fast, though they will still go flat over time - NOT GOOD.
The IPN-Pro is only reporting what it knows. In order for it to accurately report amps going into the batteries from your converter, the negative wire coming out of the converter MUST go directly to the shunt and then from the shunt back to the grounding bus bar located under the rear most dinette seat. If you look inside that area you will see that the negative (yellow) wire coming out of the back of the converter is connected directly to the bus bar thus bypassing the shunt. The converter in your trailer was not wired correctly to be able to interact with the IPN-Pro (all of the trailers at that time were wired this way.) So all the amps going into your battery via the converter after you plugged it in were not registered by the IPN-Pro and it thinks they are dead.
There is nothing wrong with the IPN-Pro, so pull the trailer into full sun and the meter will reset itself in no more than a day, probably just a few hours since your batteries are back up to charge.
I can help you fix the wiring issue if you wish.