There should be full wiring diargrams for the electrics with the manual,all hand written by previous owner on folded a4,I never really understood them fully either ,but I did use the issolator accidently once(don't touch the switch behind drivers seat)!you might have to post a pic with the tap below cooker?I think you mean the fridge,but it could be extra gas tap for a cupboard mounted gas heater?I know someone has one may be Ray?The fridge I did use on 12v only it's well worth it but make sure you turn it off at end of journey as it will zap battery, engine running only,so can be used while driving,then switch to gas if you dare I never did I found it stayed cold enough to keep bassics like milk butter,I also used a cool box.Gas before journey to get cold switch off gas at bottle too! once your packed & fridge loaded,with enging running turn on 12v small metal switch at bottom left of fridge,don't forget to switch it off at your destination. Get a fire extinguisher too..
"SYBHIL" & N0.1