Poor Gas Mileage in 2003 Caravan

I have a 2003 Dodge Grand Caravan Sport with the 3.3L. It has 30000 miles on it and a new air filter. I am getting 15 MPG while driving about 60% city and 40% highway. Even if i drive 100% city i should get 19 MPG according to www.fueleconomy.gov . I took it to the dealer and they said no codes and that it runs fine and torque conv. locks up properly. It runs great and i am using Shell brand gas 87 octane. It has 100000 mile plugs but i may pull a few and look at them but it should run bad if the plugs were bad. I guess if the dealer can't find anything wrong, i will air up the tires to 44 psi from 36 psi and only use synthetic 0W-30 oil and some friction modifiers in the oil and trans. Does anyone have any similar problems or any suggestions?

Also, i had a 2000 chevy venture before this caravan and i would get 22 mpg with same driving habits.

My 1983 Crown Vic (115000 miles) gets 16 mpg on 60% city and 40% highway and it doesn't run all that well. Also, I added greased lightning from O'Reilly Auto to my Caravan and the mileage is now 16.5 mpg.

Changed out the 10W-30 oil and used 4.5 quarts of mobile Drive Clean 5W-30 and van is bake up to 21 mpg. The manual says 5 quarts but it looks like that is too much. I also filled up with Murphy USA gas but will switch back to shell to see if that was the problem.

Usuallly non-brand gas is bad stuff.