I have recently treated myself to an '08 LS600H (not fortunate enough to have it from new/nearly new but I'm still anything but disappointed).
Whichever car I've owned in the last few years I've taken them for road trips to push the economy and have always managed to exceede the manufacturer figures once I learned the car. 60.4mpg from my Audi A6 diesel was probably my best for a 160 mile journey, and so far in my LS I have managed 35.8mpg(combined) for the same 160 mile route. I should be able to get a little more with the warmer weather approaching but I don't expect a huge increase.
I do enjoy the radar cruise control, but it has a big impact on economy with the wasted energy from frequent braking/accelerating. Set the C/C to standard mode at about 60-65mph (I have the patience with the speed with my kids in the back), and when you see the need to slow ahead cancel it and let it coast until you're free to resume C/C speed. The weight means it maintains/increases speed on any noticable gradients without the need for extra power. And in stop/start traffic, keeping light on the accelerator makes sure the power is from the battery alone. I do find that the V8 kicks in very quickly with quite quite a light press of the pedal.
I do enjoy the V8 far too much to drive like this often though, although always very quiet it is still a lovely growl.