I would take a wall plug standard for low current DC. If I have a giant battery pack in my garage full of DC, why do I need to convert it back to AC to run 15 feet into my house where it then gets turned back into DC to charge my phone & laptop?
I had a boat with both AC & DC sockets, but we had the normal socket for both and the label plates showed you which was which. I don't think that solution is acceptable for the wider marketplace, the plugs should probably be physically distinct. Maybe USB-C is the solution, though I don't know how long the cables can be.
From an efficiency standpoint, the only thing that matters is how you handle the high drain equipment. It doesn't make sense to worry about inefficiencies in your 5V phone charger because its power draw is trivial anyways.
I had a boat with both AC & DC sockets, but we had the normal socket for both and the label plates showed you which was which. I don't think that solution is acceptable for the wider marketplace, the plugs should probably be physically distinct. Maybe USB-C is the solution, though I don't know how long the cables can be.