My inverter draws between 104 to 120 amps when powering the MW depending on the battery SOC
At 100 percent charged batteries it's easy to pull 120 amps
The coffee maker pulls about 80 amps
An add on inverter for whole house, could easily be pulling 100 amps running the heating element in the water heater
It's all a matter of wiring and power management
Now if the inverter is actually turned off
I don't know where his power is going
Has the alternator been tested and verified
All batteries tested
The dash/chassis a/C checked, clutched not engaged or shorted
There are many thing that can pull heavy power