Bit late in answering here but as Paul and others have said have to go cruiser route.
Im a landy man by heart but my v8 Disco1 would pull the challenge 90 about all over the country all day, it just didnt have the weight or power to be a nice relaxing drive ( if theres such a kind with a trailer), it just made driveng hard work so I tryed the 80 series lc and Im afraid to say its much better for towing the 90.
Mines the 4.5 petrol and I get more mpg towing than the disco light!!!
In regard to disco or rrc the disco is heavier not by much though, both have same engine power (v8 or tdi) so there a much of a muchness between the both, auto best for towing (personly) but be less in mpg and power.