The simplest way would be to
The simplest way would be to have a pair of sockets inside the van. One fed from the inverter, and the other from the EHU. Then run the van electrics via a 13amp plug on a flying lead which you plug into whichever socket is powered.
I used modular switches mounted to the back of a rotary control to make a change-over switch.
build you own campervan and live the van life