Electrics in 2001 Harmony

Dear All - we are newbies and have had our 2001 Harmony for 3 months so please be gentle! We are struggling to identify exactly what switches control what! On the first photo, I understand that the water heater heats the water (!) but are unsure when we should switch on the Battery Charger - is this for when driving or when on EHU? We accidentally left it switched on and seemed to have drained both leisure and vehicle battery - is this likely?

On the second photo, I understand the labelled switches but what about the larger right hand switch? Does this switch between EHU and leisure battery? (it is not labelled).

Any help much appreciated!

PS I seem to be really struggling to add photos the right way round! (sorry!)
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The Battery Charger should refer to the mains charger, used when on EHU. It should not matter - if the charger is working correctly - if it left on when the EHU is not connected. Any drain via the unpowered charger should be very small.

The additional extra switch to the right of the control panel does not look original to me. I suspect it has been added by a previous owner.

Thank you, Roopert! If I've understood correctly, each time we are on EHU we should switch this on to charge the leisure battery? 

(and, 'oh no!' to an additional extra switch!!)

When I had an A/S conversion of a similar age, I just left it on all of the time. I never experienced any problems with this, in the 10 years that I owned the van.