If you measure Inverter +ve in to battery +ve at 2V difference, then you have a dc wiring problem, same for the -ve side. If your voltmeter measures 2V difference from the inverter’s voltage reading at the same point, then measure the voltage across the DC remote voltage sensing input pins.
possibly your voltmeter is in error, the multiplus should be within 300mV of correct voltage.
If the remote voltage sensing input measures 2V low, then there may be a problem. Connect this input to the busbars/ battery via a 2A fuse.