Frost heaters in water tanks

I have an Elddis 155 factory fitted with the frost probe heaters in both underslung water tanks. They work automatically when the temp drops below a certain level if water is in the tank. We bought the van new in Sept.

During the winter last year the heaters kept coming on whilst the van was in storage and flattening the battery. This resulted in the lesuire battery failing very early in it's life. They eventually admitted this was a recall and they fitted something to stop them coming on when the master switch is turned off.

My concern is this winter. If we go camping with no hook-up I don't want the heaters to come on and flatten the battery. How much electric so these heaters use? Does anyone have any experience of 'off grid' camping with them? I don't if it's best to just take the fuse out to switch them off and take the risk with the water freezing. The tanks are insulated so it would have to be very cold for the tank to freeze I would think. Your experiences and thoughts on this matter would be appreciated.