I have a 2000 dodge dakota quad cab (4.7L / V*). My check engine light kept coming on after tightening the loose gas cap. I purchases a new gas cap specifically for my truck and the light came on again last night. WHat should I do?

Check with AZ Parts and see if they don't have a machine they can check that for you. That's the simplest way to find it. Your could spend months and never find it yourself. That diagnostics machine will find it right away.
Gillight

Check first for leaks or worn out lines going to and from the tank. This can also be caused by a bad fuel cap.

Here is a TSB that includes P0455. Sometimes leaks are hard to find. I found some I would have never found without a smoke machine. When you looking squeeze the rubber hoses crack show up better. Check the hoses good by the batter.

PO455-EVAP Contol System Leak Detected

Gas Cap
EVAP canister purge valve stuck closed
Loose hoses
Vent solenoid stuck open
Fuel tank pressure sensor
Etc

That should keep you busy for awhile

Best take it to a shop with an EVAP Smoke Machine and Professional Scanner

P0442:

Small leak detected in evaporative purge system

if i had to guess I would say that your gas cap is not on tight enough or that is is bad. chrysler has had a lot of bad gas caps that cause EVAP service engine lights.

This code is normally caused by a loose gas cap or a bad EGR valve. It is the code for the evap system on your truck. You can go to autozone and they will clear the code for you if you can not do it yourself. You may need to replace the gas cap and or the EGR valve if the code comes back up, these are normally one time things due to the cap not being fully tightened.

By "Replace the gas cap", do you mean put it back on, or buy another one? If you fuel these trucks with the engine running, or don't tighten the gas cap, the light will come on. It will also come on when the seal in your gas cap goes bad. I have had to replace the gas cap on my jeep and dodge already. It fixed the problem both times.

Check coolant, adjust drive belt, check radior core and radiator grill clean, remove and chweck thermostat, check cooling fan, check ignition timing, water ppump may be going out, improper grade engine oil.

Normally you only need to check the level in the tank under the "engine coolant only" cap, but if you had a leak and lost coolant, check that the level of coolant under the "never open when hot" cap (it should be the radiator cap) is full - up to the brim. if not, fill it to overflowing, then cap it, fill the tank to its level, start the truck, let it reach operating temperature, let it cool down again, check the level in the radiator. if it's still fine, you need not worry about it again, just check the level in the tank daily.

this may be a defective fuel injector or a defective PCM (powertrain control module).