2003 Dodge Caravan 9 to 10 miles city - 15 to 16 miles high

Dodge Grand Caravan 2003 ES model 3.8L engine - All wheel drive, current mileage 103,000.
I bought this van in June-09, everything was fine with it, the gas mileage was 15-16 mile per galon city, 21-22 mile per galon highway.
This calculation is with a known distance, not with the speed odometer, or the overhead display.
6 weeks ago I found out the mileage per galon suddenly dropped down 9 to 10 miles per galon city, 15 or 16 miles per galon highway.
This calculation is with a known distance, not with the speed odometer, or the overhead display.
I though, may be the Oxygen sensor was dead, then I tried to replace the Oxygen sensor, I removed the heat seal to get litle more room to fit the socket into the Oxygen sensor,
I saw the back exhaust manifold was cracked very very bad (caused by what I don't know), I got so exciting and told myself "problem solved". I odered the exhaust manifold from dealer
and replaced it.
To measure the MPG, I drove the van till the fuel warning light's up and filled up 5 gallons and drove till the fuel warning light's up again, all I got was 48 city miles, I filled up another 5 gallons
and drove 1-1/2 hours on highway till the fuel warning light's up again, all I got was 78 miles.
So replaced the cracked exhaust manifold did not fix the "9 to 10 miles city - 15 to 16 miles highway per gallon".
The van runs near perfect, I could drive as fast as I wanted to, I bet even top mechanics drive this van can not tell this is a problem van.
I scanned with the ScanMaster-ELM5 USB hardware, and ScanMaster-ELM, no trouble codes found.
There's no Check engine light, no roughing idle, what so ever.

I did test the back preesure to see if the CAT plugged, nope the CAT is not plugged.
I checked all around the exhaust manifold area to see if leaking, nope no leaking.
I checked the fuel tank along the hoses to the canister and to the engine area to see if leaking, nope no leaking.

Now I've replaced the following parts:
New exhaust manifold.( Of course) Still about 9 to 10 miles city per galon.
New oil and filter. Still about 9 to 10 miles city per galon.
New air filter. Still about 9 to 10 miles city per galon.
New sparkplugs, sparkplugs wires. Still about 9 to 10 miles city per galon.
New O2 sensors (before and after CAT). Still about 9 to 10 miles city per galon.(Bosh brand, somebody told me these must be genuin, after markets no good.)
New MAP sensor. Still about 9 to 10 miles city per galon.
New Throttle position sensor. Still about 9 to 10 miles city per galon.

Any sugestions to fix this.
Thank you.