Hi Burnsy,
We don't have a DC-DC Charger (had to ask my brother that one..LOL) We have a coleman hot water service which has a shower attachment on it so you can shower direct from the coleman. We did this for 1 night and the other 3 we used the coleman to heat the water in a bucket and then just used a 12 volt primus shower that we plugged into the rear of the car (we have a 12 volt connection in the back that runs from the second battery in the car). When we used our camper with a 80 ltr waeco, we used to have an extension lead that we would plug from the cars anderson into the camper that would take charge from the cars 2 batteries as well.