Hey Scottie- Thank you ahead of time. This has been a struggle for me. 

2011 Chevrolet Avalanche 2wd 5.3 Automatic. Just rolled over 200k miles. 

Fuel system issue began with not being able to fill my tank up properly. I understand this to have been a ruptured vapor canistor which had begun filling the breather lines. I dumped what I could from the lines and replaced the canister. Mildly helped. The real issue arose when I stopped on a 3.5 hour trip for gas and the clear gas at the pump caught my eye. My truck being flexfuel, I thought that it would be ok to fill up with clear gas. 

At first all was good. I made it about 30 minutes up the road and then it started losing power, lurching, choking, and would not tolerate me putting my foot into the pedal and more than a feather's weight. 

I was able to pull over and shut the truck off and then use my scan tool to delete the fault codes and I was able to drive the vehicle again almost normally for a little while. But that ended quickly. Stopped and repeated steps. After that I was only able to go about 45 to 55 before the ECM triggered with fault codes again and then the issue returned once more. 

When I had pulled over one of those first times, I tried to rev the engine thinking that the lines were most likely just clogged because I began using the non-ethanol fuel. When I was doing so the vehicle was starting to choke and even sort of backfired inside the cats on me (or at least thats kinda what it sounded like, just wasnt a loud backfire, it sounded really contained and in the floor under me). There are times I can start it up no problem whatsoever, it will rev and act normal. But if I try to drive it, I can't get it to accept throttle. I can even have it range between being ok to hit the rev limiter to dying. It has died twice on it's own. One time it wouldn't start again for ten minutes. 

Electronic fuel pressure sensor said pressure was in 300-400 kpa range, initially. Another time it was down around 160-180kpa. The latest times it has been as low as 30kpa but upwards of 80kpa too. (All at idle)

Voltage at sensor on my scan tool said appx 1.5 volts, 

st fuel pump trim 1.59 and lt fuel pump trim is 2.18

Trim was steady roundabout .9-1.0. 

The fuel pump pressure drops to the 30's on my scan tool when I rev it up.

Misfire counts are up on cylinder 4 only. (It has had a stammer at idle for as long as i have had it. 2 years. I thought it might be because of the spark plugs but no, or perhaps the exhaust leak from the manifold bolt breaking off inside the block and allowing the end of the manifold to let air slip in.)

Using a Launch X100 pad3 scan tool:

I have codes

p2635, 

p0300, 

p0420, 

p0430, 

p0455, 

p2270, 

p013a, 

p0700. 

The oxygen sensor specific codes have been hanging around even though i changed the cats all and the sensors. I suspect at least one Oxygen sensor is bad from the factory. Will be changing for new one today. Small air leak in pipe connection between cats & muffler.

((How come when I pull both of my fuel pump fuses out of my fuse box under the hood it does not die?))

I can turn the key today and the fuel pump primes like it should and fires up. 

Cleaning the grounds for electrical connections right now. 

Could really use some insight about what to do next if my changing the o2 sensor and ground point cleaning doesn't remedy the situation.