What have you got on the DC system that is drawing approx 1300W.
Have you got your BMV wired correctly, only the BMV shunt battery side connected to the batteries and everything else connected to the system side of the BMV shunt. See
On the Cerbo GX have you set the Multiplus to “Inverter Only”. On the GX screen, tap on the inverter box, you should see a menu item “Mode” at the top, this should say “On”, if it says inverter only you will not get charging, so click on that and change the selection.