What would cause your 1999 4runner to get 10 -12 mpg?

If you have no trouble light I would suspect an injector or the cold start injector of dumping fuel. Do you have an exhaust leak? If so the O2 sensor will get the wrong information and dump more fuel. How is it running? Rough, no power, slow to accelerate? Check the filters and intake manifold for clogs in the tubing and vacuum hoses. Even the permanent ones can get clogged and not allow vacuum to key components.

If there is a check engine light on do a diagnostic check to find the code and it could point you to the exact component. If need be pull into an auto parts place. They usually do the check free of charge. You just might have to sit around for a bit why they get to you.