In case there wasn't enough confusion:
http://www.caravanclub.co.uk/media/1022 ... ing-mo.pdf
Personally I don't like the camper pulling any 12v supplies from the truck for any reason if the engine isn't running.
I can understand the 3-way absorption fridge having a separate switched supply so its 12v heater is only run when the engine is running and is not accidentally drawing off the vehicle or camper battery at other times - hence the need for a separate wire for it.
If there were only one switched supply from the alternator and that supply was wired to both the fridge AND the camper battery it would mean the fridge would run from the camper battery when the engine was off
Hence if you only had one relay (and it was single-poled) you would want that relay on the wire powering the fridge, but not on the wire going to the camper battery.
Steve.