The magnum is a combo unit
Charger and inverter
If you unplug shore power and the TV and outlets still work
The inverter section is operating
If you are plugged into shore power and a meter reading at the batteries is higher than 12.7 preferably over 13.0 then the charge section works
Converter aka chargers charge the batteries
Inverters use the batteries to make 120vac
Combo units like your Magnum do each function, but only one st a time
Not both at the same time
They really should be called "charger-inverters" or just Combos
It would be less confusing
Especially to new users