Chrysler Repair: 04 T&C 3.8L fuel mileage problem, fuel mileage, gas mileage

Question
We are at our wit's end with this van.  It constantly reads low battery at startup because of all the electronics and is recently unable to start because the battery goes dead.  The dealership says if I'm going to be loading the van for 10 minutes with the doors open, leave it running so it won't drain the battery.  Nice solution.  Also, about the time we started exp. battery problems, we lost gas mileage.  It dropped from 21-22 mpg down to 14-15 mpg.  There's no outside reason for the drop that we can find (except for the first instance, we used gas from a station we don't normally use and I think it may have been contaminated).  The first time it dropped, the check engine light came on and there was a code indicating the emission system needed reset.  Once reset, we were getting 21-22 mpg for about 3 weeks (yes, we are meticulous about checking our mileage against the electronic reading and it's really close).  After 3 weeks, it dropped again to 15-16.  After another week, it was at 14 and still dropping.  We took it back in and had the computer reset.  Mileage returned to 22 mpg.  This has recurred about 4 times now.  The last time, the dealership told us they wouldn't keep resetting the computer if there wasn't a code.  We're thinking about replacing the plugs and fuel filter.  Is this a good plan?  Also, where might we find the filter?  I've been told it's outside the tank but i'm not sure where.  Any suggestions or insights on this ridiculous problem would be greatly appreciated.  This van is only a 2004 with 20,000 miles on it.  The problem started at 16,000 miles.  I would think 4,000 miles would be enough to rid the system of some bad gas.

Oh, and we've always gotten high fuel economy on our vehicles because we drive easy and we work at getting good gas mileage.  This problem is not due to driving habits.  I do think we may have gotten a batch of bad gas prior to the first occurence of this problem.  What would bad gas do to the fuel economy and the system as a whole?  Also, how do you "repair" the damage after running bad gas through the system?  Thanks.

Chris

Answer
hello chris
you have got bad gas . if you were to come to my shop i would change the gas filter it is under the right side of the van along the frame rail . then i would drain the tank and clean which would be removeing the fuel pup and reinstall. and then tune it up with plugs and wires.

and clean enj system
Good luck to u hun and u have a pretty name and if you need more help let me know pete