Range Rover V8s should do late teens to high 20s.
The diesel when chipped (£500 or there abouts) is much better, ive seen over 34mpg from one.

What about a Discovery 3?
The tdv6 hse is a great car and in auto can do 36mpg in general motorway driving, ive seen over 40 from them. can also be chipped gives 50bhp and 120nm of torque.