2006 Pontiac Montana SV6 - What to fix first?

I have 5 different codes being displayed.. P0107 (map sensor), P0405 (EGR valve), P0452 (fuel tank pressure sensor low voltage), P0453 (fuel tank pressure sensor high voltage) and P0641 (5 volt reference voltage)..
My gas gauage is malfuntioning and I would like to know the best plan of action on this.. Do these all tie together in some way? Will I have to replace everything in the line or do trial and error? Replacing one at a time until the issue is resolved? I just had my transmission replaced about 3 weeks ago and a week later, the gas gauge began to just fly all over the place. Full to empty, 1/2 tank to empty, etc... Thanks for any advice

I seriously doubt the tranny work has anything to do with your fuel gage....

But were the other codes there before or after the tranny work?

It's possible the P0641 could be causing the MAP, EGR codes......need to check both for the 5 volt reference voltage.....could be a bad PCM or a wire may be pinched somewhere....

Fuel gage problem could be a bad sender, or the pass-through wiring harness, that is located just behind the driver's seat and passes through the floor, and then goes to the fuel tank......pull this connector, underneath the vehicle apart, and check for corrosion in the contacts.....

Codes didn't show up until a week after trans rebuild... When the gas gauge started to malfunction.. I took it back to the trans shop, they visually inspected and pushed on electrical connections under the hood and moved the ground wires around that attach to the engine block. They cleared the codes and couldn't get them to come back. 2 days later, the engine light came back on and the same codes came back. I looked at the connector you mentioned on the underbody, and found no issues.. I will get my multi-meter out to check for these reference voltage in all the locations. Could flashing the PCM fix it? Just starting to grasp straws here.. Thanks for your input!!

I'm just a little suspicious that this happened after a tranny removal/install....

One thing you can do with the PCM, is remove it(I think it is in the air box).....then with the car running, lightly tap the top of it, to see if this causes the CE light to come on....also check if you have a mouse nest in there...may have eaten some wires...