a tune up normaly helps with your gas mileage. filters, plugs, clean the injectors, maybe a distrobuter cap, wires if they look crappy, and clean the throtal body. and if you check light is on find out way and fix it.
Other possible factors include tire pressures, driving speed, oversize tires, extra weight in the vehicle. If it is a relatively short trip (maybe one fillup) differences in gas pumps can cause erroneous calculations. I know of a couple of pumps near here that were about 10% off for several months until the state inspectors caught them.
As mentioned above...don't get too crazy with the idea something is drastically wrong. A good tune-up, maybe even a new o2 sensor may be all that is required. INSIST on Genuine Toyota PARTS!. They are the best parts in the world for your truck, regardless of warranty length.
Cleaning the throttle body is great idea. I like to use fuel injection cleaner. Not the spray stuff, I mean the stuff that you are supposed to put into your gasoline tank. It does not evaporate rapidly like intake cleaner sprays. I do use a spray cleaner to finish the job.
Thanks for the advice, I used some Seafoam injector cleaner and it worked great but haven't noticed any improvment on the mileage. The truck did have alot of crap in it for the trip, and i am running 31s, the tire pressure is fine. but I think it is probably due for a good once over.
A product put through the injectors will not necessarilly clean the hard brown ridge of crud that forms where the throttleplate rests in the throttle body. Cleaning this area can only help, and some people find the engine idles faster after cleaning. Adjusting this is a snap.
You may also have a dirty air filter or feul filter with can be causing you to apply more throttle for a regular job. Does your timing chain rattle ? If it does than you may have a streatched timing chain wich will cause a loss of power and cause you to apply more throttle. Thats whats going on with my truck right now. Chain is streatched and I have to go up slight inclines with almost full throttle. But if u dont notice a loss of power of acceleration or a clicking noise from a rattling chain than thats not it.