This piece of gear is certainly a win/win. I will certainly be fitting one and swapping my existing storage heater out. There is a kit so it will fit in the same entry hole so no structural damage to the camper. It will create a couple of cubic feet of extra storage space inside the camper (always welcome). It will weigh nearly 30 kg less (fuel saving and handling improvement) and it will use only about half the gas, so that will also last longer. It will need me to remove the fridge in order to fit it, but I need to do this anyway (would you believe?) in order to swing the door the other way (which will be much more convenient): http://www.adventurerv.net/girard-tankless-propane-water-heater-p-27164.html
PS: Some people have had issues with these units whilst others love them. Problems could be related to older units without dial control, installation, or failure to read the instructions, etc. As with any hot water system the unit should be fitted as close as possible to the hot water tap in order to save water.