Your van is newer than mine so some of the electrics will be different. However from what you you say it sounds a though your battery charger is not working. Normally on hook up (assuming your external electrical point is live, your cable and connections are good and your MoHo ECB is on ) you should be seeing about 14.3 volts going into your habitation battery. After your battery is fully charged and you disconnect the EHU the full voltage will settle at about 13.5 volts after a couple oh hours. dependant on your battery type.
If your battery charger is charging ok and your battery is loosing voltage over a couple of days, you either has something switched on that is drawing power or your battery is in need of changing. Have you checked that the light in the garage or Waldrobe have not been left on for example?
Another issue is that lead, gel or AGM batteries should not be discharged below 50% or they will be damaged and their life severely shortened. Therefore if the batteries have been often run down or left in a discharged state, they may well be finished.
Final though, do you have a volt meter and an amp meter. They make calf finding so much easier.
Hope some of that helps and I am not trying to teach my Grandmother etc etc.