Toyota Repair: Toyota Sienna 2004 gas mileage, toyota sienna 2004, gas mileage

Question
Have consistently gotten 15 mpg (99% stop and go city driving) for 4 years.  I took the van in recently for a 60K major service and since then, have been getting 12.4 mpg which is pretty significant on a 20gal fillup.  This has been going on for 4 tanks so far.  Dealership did its computer check and said everything is fine but it's not.  Have been filling up at a variety of different gas stations to see if its one brand of gas but still get the same bad mileage.

Answer
Hi, can you tell me what was done on this service/ check if spark plugs were replaced with same as the originals. this is hard to tell. On this car averything is computer controlled. Is you engine performance the same try to notice any small difference.