Swift motorhome electric problem

mr shifter 1

Hi my name is chris I am a newbie wondering if any one can help   I have a  2005  swift 590 rs m/home  when m/home is stored vehicle battery is fine starts after 4 weeks.    When  on site hooked up to electrics  if I put the rocker switch on control panel
to the centre so I am just running off mains it seems to drain the engine battery so I have to switch the rocker switch to engine battery to charge it for 5 min  then it will start  Dont know if I am doing it wrong ie switch in the wrong position  does not
say in manuel  which way switch needs to be and the chap I bought it of had a set up note stuck to the wall stating what to do when arriving on site   thanks Chris