Flat liesure battery, why. beware cold weather.

Warning, Cold weather- Perfectly good, leisure batteries {2 x 130 ah) suddenly completely flat overnight, despite being well charged.  Could find no fault, but 5yo batteries so replaced,   Same happened again two weeks later, but caught before batteries ruined.  Spent all day in investigation, found no problem.  Watched for days, no problem, Set off down into France, then, middle of the night, alarm, no power, flat batteries.   Have owned for 7 years (Bought 2 year old at 7k miles), never had a problem before.  Checked everything, spent ages, got dark, got cold then suddenly noticed big resistance drop on avo.   Switched everything off, still big earth, removed all fuses, still big drain.   Started chasing, found non fused wire, bed out, lockers emptied following wire.  Eventually found it, An added 20 amp water heating element in the water storage tank, with a controller that cut in at 3 degC. Not in the drawings, not though any fuse, no switch, obvious ad-on.    OK if on mains power, but without battery charging, batteries quickly flat.   I may be the only one with this.  Just thought I would warn. But would be nice to know If I was not the only one caught by this.