I don't know much more than what I had already stated. Got the vehicle from a family member that recently had the fuel pump replaced. That'* when the problem started. Engine began misfiring. Hooked up OBD2 scanner and came up with random misfire and downstream O2 below threshold. After a lot of googling and YouTube videos I replaced a lot of stuff like I already mentioned. Funny thing is that half of the time when I replaced something, the engine would run fine for a while and then start acting up again. Took it to the mechanic and first mechanic said he didn't have the equipment to properly diagnose it and gave it back to me free of charge. 2nd mechanic said they ran the full gambit of diagnostics on it and couldn't find anything. Suggested to replace the "cheap copper spark plugs" when the spark plugs were new platinums. I obviously gave them some words and took it to 3rd mechanic. They said wires were pinched by fuel tank because they weren't placed in proper clips when fuel pump was replaced. Gave it back free of charge. Ran fine for a while and then began acting up. I dropped the whole tank and saw the complete hack job done on the fuel pump replacement. They even cut the top of the tank to replace the fuel pump which it'* obviously completely unnecessary. I replaced the fuel pump and the whole tank. Still misfiring. Took it back to the 3rd mechanic and they said the problem was in the engine wiring harness, but they couldn't find any damaged wires. They had a very difficult time finding the correct harness and ended up getting the wrong one multiple times. They said they could retape the harness and try that out because the only correct harness they could find was in Florida for 1,300 and that'* obviously way too much for a harness. As to where specifically they retaped I'm not sure. Might be my next project to figure that out. Beings I just bought a place with 25acres I have a lot of other work to do besides working on this Avalanche that I'm beginning to get burnt out on .

A bit of data would be helpful with this. Fuel trim, O2 , ect. As the others stated I would have a hard time justifying a harness replacement. Can you take it to a Dealer for diag? As they would have the equipment to diag your issue. A skewed sensor reading due to sensor or wiring can give you the p0300. As I understand it actually has multiple cylinders misfiring. As P0300 can be set by a single cylinder also. O2 sensor 1 readings will effect fuel trim and could cause a lean misfire an multiple cylinders for example. If the issue started when a repair was made I would start looking at that area. first. I know this isn't much help but again if you can provide as much data as you can, There are some pretty knowledgeable people here that are willing to help. Take care.

This is a lot of great information, especially:

What is the fuel pressure with key on engine stopped, and also with engine idling?

Can't remember exactly what it was but had the correct amount of fuel pressure. 40-60+PSI I believe

Here'* what I see:

1. Bazillions of GMT800s were built over six model years prior to yours. These were in the forms of Silverados, Sierras, C3s, Tahoes, Yukons, Avalanches, Escalades, Suburbans, YukonXLs, EscaladeEXTs, H2s, 2500 versions of the Suburban/YukonXL/Avalanche, and 2500/3500 versions of the Silverado/Sierra. Most of those had gasoline V8s from the same family as yours. During some or all of these years, the wiring harnesses of these (as related to the engine) shared a lot of DNA with V8s in the Camaro, Firebird, Corvette, Express vans, Savanah vans, some Rainiers, some Trailblazers, and the SSR.
2. 16ish years ago the car was built at a factory somewhere.
3. Lots of stuff happened over the years to all the cars in step 1 as well as all the cars built in the same model year as yours.
4. CathedralCub never heard of a wire harness needing to be replaced on any of these to solve engine issues, let alone without a specific diagnosis of (a) bad wires in it. Sure, CathedralCub has heard of the occasional wire break and/or damage. These are commonly fixed with methods and sourcing identified by several in this thread as these are the common ways to fix the extremely-rare wiring issues on these (and many other) engines.
5. Something caused a family member to say something like "Why yes I'd love nothing more than to replace the fuel pump in this car."
6. The fuel pump was replaced by savages. The same savages damaged the fuel tank for reasons inexplicable to non-savages.
7. All of these problems started.
8. The car became yours.
9. Lots of unrelated parts were replaced because of step 7'* problems. None of these replacements permanently fixed step 7'* problems.
10. Mechanic 1 unable to properly diagnose.
11. Mechanic 2 claims to have diagnosed a lot then attempted to swindle you.
12. You replaced the fuel pump and tank.
13. Mechanic 3 claims "there'* yer problem" but can't actually point out where the problem "there" actually is. This is understandable in circumstances with non-servicable parts like the sealed PCM or the guts of the infotainment stack, but not in wires that are eerily similar to the wires that have run through engine bays of all manufacturers' cars for decades. How do they know "the harness" is bad if they either (1) haven't done a continuity test on each conductor all the way to the contacts on each end or (2) if they haven't tested at all . . . ? From what I understand of this, this is a guess on Mechanic 3'* part.
14. Internet advice was sought.
15. Here we are.

I don't see any actual proof related to the wire harness. I don't see why they have said "the wire harness" as opposed to "the fuel system or "the engine" or "the ignition switch" or "the headlight ****". Without this, there'* no need to go chasing an obscure part that nobody has, only to replace it at great expense and then later find out from Mechanic 7 that it was the fuel filter or the tailgate latch.

Speaking of which, has anyone put a fuel filter in it?

Next step in my book is to understand what the fuel pressure is now. Not guesses, but actual readings from a fuel pressure gauge attached to the fuel rail.

Can you confirm my vehicle has a fuel filter aside from the sock filter on the fuel pump? I went chasing a fuel filter because that seemed like a likely culprit, only to not find one. I can retake fuel pressure tomorrow. But I specifically remember taking it a couple of times and it falling under the appropriate fuel pressure for my vehicle. I should have inquired into what exactly the mechanic found wrong with the wiring harness. When I went in, he was not there and I was in the middle of buying a house so i didn't delve into it as much as i should. Was just satisfied they got it to run better.... for a while.

Fuel pressure is 61PSI at idle

Good point. If it has one, it will be along the frame rail on the driver'* side by the driver'* door.

How was this pressure read?

Is this key on engine stopped or engine idling or both?

fuel

Fuel pressure with engine off was 55 and with engine on and idling 61