Hi Paul you don't say wether the Shoggie will be diesel or petrol, LWB or SWB - all this will make a slight difference to MPG if you are really that bothered.
My 2000 3.5 GDI SWB (petrol)does about 25 mpg average and 20 towing a 1.5t van.
I took it to collect a one tonne trailer the other day on a 200 mile round trip and the fuel computer finshed at 28.5 mpg which I suppose is OK. Can I just say that for me, MPG on these kind of vehilcles is not a consideration factor - any car weighing in at over 2 tonnes and having a transfer box and extra transmission components is going to be thirsty on petrol or derv so you have to face it.
By the way has anybody noticed - the price of diesel seems to be equaling that of petrol in many places, when it was a few pence dearer the economy advantage was questionable, now however it will maybe cheaper to drive an oil burner? Not so much fun though.