City gas mileage too low.

Hello guys
2003 Grand Caravan 3.8L, ES model AWD, 105,000 miles
Driving highway I got 20 miles per gallon, but driving city I can only got 10 miles per gallon, if I drive the "81 years old man mode" I got 11 miles per gallon.
Friend of my has same exact model but the mileage is 120,000, he got 15 miles per gallon city, and 20 miles per gallon highway.
I did switch the O2's (2 of them before and after) from his van and drove the van for 150 miles(city) to see if any differences, but nothing changed, still 10 to 11 miles per gallon ,and his van got same gas mileage as before.
I just bought this van 7 months ago, because I saw the van of friend got 15 city ,20 highway is not bad for a 3.8L big-O heavy van, but mine now 10 city is too low, 20 highway is not bad.
Problem now is 90% city driving, and only 10% highway driving, I really want to keep this van for a long time, so do you guys have any ideals to fix it?
Thank you.

Could it be this simple?? Our 03 was getting 15 around town. I added Sea Foam to the fuel tank, 2 fill-ups in a row. Now it's averaging 19.5 around town. Haven't done anything else to help, so it must be the Sea Foam. I think it was around $8.50 a can at Walmart.

I added Sea Foam to the fuel tank (http://www.automotiveforums.com/vbulletin/#)
Thanks Brix
I'll try the Sea Foam, if nothing change ,I intend to replace the Air Charge Temp sensor, do you think that helps ?, I did relocate the Air Charge Temp from the air inlet tube to outside the tube close to the head engine for the sensor to get more heat and see if anything change, but so far nothing change.