12VDC is the standard on boats, campervans, RVs, etc because it can be delivered straight from the batteries. In order to deliver 240/120VAC you'd need an inverter to generate the signal, and this obviously introduces loss during conversion. For cable runs through the length of my boat (52ft), the cables aren't particularly thick - the thickest cables are in the engine bay, going from the alternator on the diesel engine to the batts, and again from the Solar controller (MPPT) to the batts.

There's a plethora of stuff you can use with 12VDC to the extent that the only things I run off 240VAC is the vacuum cleaner and the TV (12VDC TVs are available but the cost vs quality payoff isn't really worth it).