Deck mount converters have a cord to plug into 120v. You just plug into the Honda via an extension cord.
If mounting the deck mount up front you will need a 120v receptacle to plug it into. In the 5er we had, I extended the receptacle that was on the front wall low down beside the steps up to the bedroom area.
That receptacle had its back into the cargo bay up front so I just ran another receptacle from the back of that one up to the front of the cargo bay where the cord of the deck mount could reach it.
The OEM 12v wires from battery to DC fuse panel stay as is and you just put new short wires from the deck mount to the batteries.