Toyota Repair: RAV4 mpg, rich fuel mixture, air fuel ratio

Question
I bought a 2002 RAV4 about 2 months ago from a toyota dealership.  They had no maintainence history on it from the previous owner.  A 70,000 mile checkup was performed on it by the dealership.  The mpg in town is 25 and highway is 23-24(it should be at least 28).  Most of my driving is highway.  If I mix in town with highway driving on a tank of gas I get 25 mpg.  When I asked a mechanic at the dealership if the problem could be the fuel filter, he was adamant that the check engine light would come on.  It was suggested that I did know how to figure gas mileage.  So I requested a tune up even though the spark plugs were 120,000 miles.  I was also told that some vehicles just do this and nothing can be done about it.  I've owned two other Toyotas--that's why I bought this one since they tend to last a long time and the mpg has been low.  This is a RAV4 L 4x4.  I checked carfax.com. The report showed 26 items.  I didn't pay for the report because I thought the items could just be maintainence and did not want to waste anymore money on it if nothing can be done.  Do you have any ideas on the low highway mpg?  Did I just buy a lemon when it comes to highway mpg?  Everything else seems OK.

Answer
The fuel filter is located inside the gas tank and is not normally replaced, the fuel filter won't cause low mileage and even if it had a problem the check engine light would not turn on. The check engine light will turn on if the computer detects a malfunction or an errant input signal from one or more of the many sensors on the engine and transmission but that doesn't seem to be the case because there is no check engine light on.
If there was a significant amount of deviation of fuel consumption it would mean that the engine is running in a richer than normal air/fuel ratio and this would turn the check engine light on, this can be verified by connecting the diagnostic scantool and doing an active test while driving on the highway, an exessive rich fuel mixture will show up on the fuel trim display,the toyota dealer should be able to do this and provide you with a printout of all of the engine parameter readings, it can then be determined if there may be a problem.