12 volt system

Looking for some advice. During the last couple of trips when we have been on E.H.U. there have been a couple of occasions when the 12volt system has for no apparent reason dropped down i.e. dim lights,radio switches off  etc. If I then flick the car/van switch the voltage returns to normal. The leisure battery is brand new heavy duty and holds its charge so no problem there. This seems to be an intermittent fault happening at odd times. Has anyone any ideas on what could cause this please?