In my experience of wheel bearing problems before (had none on the Lexus though) they will make a droaning noise, but never had one that caused shaking. If it has gone that badly to cause shaking, then as suggested you should be able to wobble the wheel all over the place quite easily once jacked up. Infact, I would expect to be able to feel the play if just pushed and pulled the top of the tyre while it was not on a jack too. I would check for dented rim on the wheel as this can cause very evident shaking if bad enough too. A binding brake calliper is also something to check. Mine had a binding calliper on the rear and that caused heavy vibration and noise untill the pad was essentially worn off again to stop it catching. Easy fix without needing to replace the caliper. Also check that the wheel is actually on properly too, sounds stupid but my brother booked his Rover Sterling in for a wheel bearing/CV joint problem only oto be told by the garage that his wheel was loose. I haven't let him live that one down yet
As far as fuel consumption goes, it sounds a bit heavy to me. I get between 25 and 29 on mixed driving and not had under 20 with just town driving. It does of course depend how enthusiastic you are with the accelerator though. I have 235/40 18's on the front and 275/35 18's on the rear of my '96 GS300 Sport so the tyres shouldn't be causing you the problem. Maybe just a good service required on it.