The relay under the bonnet is probably for the fridge which has its own power from the car battery.The leisure battery will have a 25 or 35amp fuse which should be fitted very close to the leisure battery itself,if all is ok check fuses on the power unit,there will be 3 or 4 usally,if there ok and you are getting no power when switching on the 12v then fault usally lies at the switch itself.even if you are getting 12v to and from the switch it can still be faulty,because of the amps drain.clean contacts on the switch,but you should take out the fuse for the car and leisure battery if removing the panel for safety reasons.you can also check for 12v at the rear of the unit if you have a wiring diagram,if its a zig unit usally number 2 is the car battery and number 4 is the leisure battery.all odd numbers are earths,
mikec
May 2009