since we took delivery 3 weeks ago.
So it's a new boat?
Why you don't ask the dealer, answering questions and solving problems is/should be part of the after sales service.
As you could crank the engines then i don't think that the batteries are the problem with your electric steering problem.
A total blackout of the steering system seems another problem to me, probably a blow fuse.
am not sure if the electric steering is on the house battery or the engines batteries, as it is a integral part of the engine i would connect it with one of engine battery to make sure that both work or not. Basically, if the engines run then the steering should be operational as well.
Do you have (as me) two battery selector switches behind a door port side on the transom and set them to 1 and 2 when using the boat and after use setting both to Off?
Do you have 3 batteries or more in the battery compartment?
Chris