Hi, I have a disco 4 towing a 16-49-3b and am in the same process of trying to work out what to do. So far this is what I've come up with.
The round 7 pin land rover to 7 pin flat cannot power the fridge, the wires are too thin. I used to have pin 2&3 power the fidge in my previous tug, a Toyota.
It is possible to buy a narva 7 pin round plug (super cheap auto, eBay etc) and wire up an Anderson plug and us the white aux power out ( need to remove centre pin in the narva) in the back of the disco 4, BUT, there's always a but, it depends who you speak to as to what that aux plug can supply. Speaking to LR here in Sydney they are quoting a figure of 40A that can be supplied from that plug, they have said it might also be 30A and I've also been told by another D4 owner that the aux plug is protected by a 10A fuse so that would indicate a max of 10A. Yeah I'm confused too. Also the wiring between the alternator/battery to the back of the plug is an unknown and it has been suggested that it's pretty thin, if anyone knows for sure I would luv any input. Problem here is the voltage drop issue that Glenn has mentioned above.
I'm thinking that Glenn's setup would be ideal, ie separate Anderson Plug the rear of the car to supply the van. The alternator in the D4 is supposed to have quite a high output, 180A max with about 47A at idle would you believe. This is required to make sure all the electronic wizardry still works in the D4 when it's idling at the lights. However this would mean that if a descent cable is fitted between the car and van we should be able to get a descent charge into the van batteries.
I was thinking a single Anderson plug to charge batteries and I'm hoping with some more research I can use the land rover aux power at the rear to power the fridge. If all this doesn't work I'm thinking to power a ctek D250 dual via the Anderson plug to use as a DC-DC charger and also use the same Anderson plug to also power the fridge.
Cheers
Mike