It will be something really easy to fix or..............
Try all the simple things first - all lights off, fridge switched over to the 240V position, water heater off, etc. Try disconnecting the battery and listen for a "click" of a solenoid opening (power off), its possible to have a solenoid in the power on position and taking power all the time, someone may have fitted the wrong solenoid.
If you cant isolate the problem from the usual culprits above, then my guess would be the dual battery charging circuit is playing up. There is a little "black box" which controls the charging of the vehicle battery and the leisure battery (vehicle battery reaches full cahrge then the leisure battery charges), when that fails it allows charge from the leisure battery to either flow back to the vehicle battery or to some other power consumer on the vehicle circuit (rather than the camper circuit). Time for a visit to an auto electrician.