Charging Problems Freedom 458 Inverter/Charger

You will need to follow the voltage toward the batteries. use a test light is the easiest way.
Start right at the DC terminals on the inverter and follow the positive to the batteries, then do the same with the ground.
It could be a bad connection. Some of the crimp connections covered by shrink tubing could be bad, if equipped.
Just because the display says it is working, you know it is not reaching the batteries if they are not getting charged.