What do we call you , Man, About or House???
I suspect you are describing a cigarette lighter type adaptor. These take the 12V from the cars supply and regulate it down to 5V for the USB equipment. So it is possible to use the 6way hub, but you would still need to use the 12V adaptor. The simplest way would be to get a cigarette lighter type socket (Most good motoring stores should have them) and fit that in a convenient place in the caravan, and the simply use the hub in its original form.
I would be wary though. Whilst all the devices you mention may only be low current, they tend to be used over extended periods, so the power used starts to add up. As I posted in an earlier popst, if your off grid, you have to far more circumspect about what to use. By rertaing the hub in its original form yoiu have the option to transfer it to the car for charging items when your out and about.
ON your other thoughts, yes changing all lighting to LED will certainly use less battery power, and of course don't forget that many of the standard appliances will use gas, so you can still have hot water, heating, and fridge working. Solar panels can help to reduce your dependence on mains and extend the off grid usefulness of a battery. Having a second battery that is charged in the car can allow you to swap them over, or of course you may consider a small generator, but make sure you go for onw with an inverter output to protect your caravan electronics.
For a really expensive but ECO solution you can get Fuel Cells.