Autozone should be able to clear the light or you can do run your
truck through a driving cycle and it will clear itself.

A driving cycle is a series of moves the computer must see before it
deems the fault to be cleared.

http://www.dakota-truck.net/UPGRADES/driving_cycle.html

It could be either case...  By law your MIL must be capable of being
extinguished using any OBDII scantool.  Either Autozone didn't clear it
for you or the problem still exists.  Not knowing the year, engine and
specific code(s) will yield you less than specific or helpful answers,
but in general gas cap failure is common on newer Dakotas.  I'm sure
the Autozone employee tries to sell a gas cap on any OBDII evap leak
failure code, so it was likely a coincidence that he was right the
first time.

You could have a leak at the gas cap, purge solenoid, leak detection
pump, fuel tank, canister, filler neck, or vapor pipes/hoses.  A
Chrysler DRBIII scantool has a function to run a quick evap monitor
leak test and report pass/fail.  If your MIL doesn't go out soon it
would be wise to spend some proper diagnostic money with a capable and
equipped tech.

I wouldn't use a one size fits most Stant-o-matic Autozone gas cap,
either.  Call a Dodge dealer and ask if any improvements have been made
to your gas cap design (they will be able to tell by part# or
supersession details) and get one if it has been redesigned.

Toyota MDT in MO

For an issue as noncritical as a small evap leak it depends on how much
money you feel like spending on it. If you are rich and don't care
about price then just take it to the dealer, but if there are no
noticeable gas fumes or anything else objectionable I would just pull
the cel bulb or hack the evap sensor to turn off the cel light.