I replaced my battery 3 months ago. Now dead

Remove negative cable from battery, connect a test light to negative terminal, opposite end off light to negative battery post. ( let sit for 3 to 5 minutes). Then start pulling fuses, when the light goes out that's your bad circuit. God-Bless! Its worth a try.

Diagnose the door lock issues first,that you know is a problem

Why would you replace the alternator ?

i believe there are 2 alternator's for that 5.3 i think you need the one with the higher amps. I had the same problem and went with the higher amp alternator and i have not had a problem that was 3 years ago.

If you have a drain on your battery , it would not take 18 months for the battery to go dead . What do you mean The battery gauge in the dash shows only about 1/2 power, When ? When the vehicle is running ? Not charging ! The battery gauge as you call it has nothing to do with the Battery . That's showing how much the charging system is putting .The volt's indicator is controlled by the PCM - engine computer . With the engine running should show 14.1 volts .if your only seeing 7 the charging system isn't working. The alternator is controlled by the PCM as well ! Have it check by a GM dealer .

Remove positive battery terminal from battery & put a 12 volt test light in line between cable & battery post. As long as 12 volt light is illuminated, you have a draw. Start pulling & re-installing fuses, one at a time,until you see it dim or go out. If it goes out, you found your draw. If it dims, slight draw, such as radio memory etc. if all else fails, make sure you know where they all go, then pull them all, and start putting back one at a time. When light comes on bright, you found the circuit causing the draw.

Regardless of the battery age,sounds to me as if you have a drain on the electrical system First thing to do is completely recharge battery. An initial quick inspection might show a light staying on, whether it be a door , reading, glove box, or trunk lights, among many other things. If nothing is obvious an electical reading of the system while removing and reinstalling fuses will show the circuit causing the drain, Hope this helps

the battery is located under the rear seat, check battery connections. if connections are tight replace starter.

Take nozzle of WD-40, stick it in key slot as far as you can, lots of spray in there and keep spraying as you pull little red nozzle out slowly, and give it a little time, and try working key, first just slide it in & out a few times then keep trying to turn it. May take 2 or 3 sprayings to unseize things.

You seem to have an electrical drain on your battery from what you are saying. However the drain can be coming form you alternator. With some skills I can tell you how to check for the source of this drain, Its an old technique that I learned years ago. Get a regular test light(the one with a bulb/light in it) disconnect your negative batter lead from the negative post on the battery. Connect clip end of test light to battery pole. Touch pointed tip to battery cable(negative one) With ignition key off and out of ignition and all doors locked and ALL OTHER ACCESSORIES OFF... this test light bulb should NOT be lit. If it is there is a drain. Go to main fuse boxes and remove fuses on at a time(MAKE SURE ALL DOOR ARE LOCKED AND ALL ACCESSORIES ARE OFF when doing this procedure) As you remove a fuse if light is still on replace in the same spot and move on to next fuse until light on test light goes OUT. when it does "Bingo" that's the circuit that has a drain in it. Some accessory in that circuit is draining your battery. Locating the source should be easy the fuses are marked by what they control. Contact if you need more help. Good Luck