question

It's implicit, but I'm assuming you're in 120/240VAC split phase configuration for North American power.

Please note that the Multiplus II 2x product was released to address this issue - automatic handling of switching between single and split phase shore power. They are still single phase inverters, but they intelligently handle shore power. If only input on L1 is detected, L1 is shorted to L2, and both legs are fed with the same sinjgle phase 120V. If split phase input is detected. L1 is fed through the inverter to charger/loads and L2 is powered directly by shore.

Standard 30A to 50A adapters short L1 to L2 to provide both legs of the RV panel with 120V. This means the Victrons' L1 and L2 inputs are shorted together. I think you can imagine that this is not good.

The easiest solution would be for you to create a custom 30A to 50A adapter that ONLY passes L1 to L1 and leaves L2 open.

Then in the VE.Bus System Configurator:

You need to uncheck the "switch as group" box.

This will allow the L1 inverter to properly sense, synch with and pass through L1 for loads and charger while L2 continues inverting. Provided L1+L2 loads do not exceed your L1 charger capability, there should not be a net battery drain.