*horrible fuel mileage no low end torque

Hello all, I know it's not LS1 but LS based and you guys are usually more help than other forums. This is all about my 06 5.3 4l60e Silverado 4wd. I'll start with what I know and what I've done.
Past 2 yrs my mileage has been dropping from a whopping 20-21 to 13.5, this is 90% highway- 10% city. Roughly 8-9 towing a 3k# camper.
So I started last year when it was around 17mpg with a tune up, (factory plugs Ac Delco41-962,air filter, ran some fuel injector cleaner)
That didn't do any good so I moderately drive it around town. So far this yr I've done all 4 02s, fuel pressure around 55-60 key on, roughly 52 or so running. I'll have to double check on the pressure though I just remember it was spot on with specs. Changed rear diff fluid, cleaned mass air flow. Flushed coolant with new standard thermostat and coolant temp sensor. Oil is Changed religiously between 3-3500.
I used a snap on scanner for some of the standard tests they have. 02 sensors m/v is where they should be. My fuel trim bounces between -5 to +5 which I assume is good. My MAF readings are around 180-200 g/s at wide open throttle. My self and a tech who is one of the most knowledgeable people I know said that reading though the scanner it looks very healthy. Looks good to me to.
I've never changed coils or anything, but watching each cylinder there were no misfires ever detected and I watched each for a few minutes. I also did a fuel injector cut out test, each one dropped rpms down 50-75rpms at idle and a 100 rpms or so at 2k. Compression is at 155-160psi all cylinders.
I don't feel my cats are plugged because I have a fairly good exhaust note and plenty of pressure at tail pipe.
My buddy said my truck smelt rich a few times. I've smelt it at idle a few times. A have had a black haze of smoke out of tail pipe a time or 2 at HARD throttle. Which is rare.
Any ideas or suggestions would be appreciated. I'm missing some things I've done I know. I just haven't wrote down all of it. The fuel mileage and no low end torque is my biggest issue.
If it's any help not that I try to one wheel peel, my father in law used to punch it at a light and it would light up the tire and it does not have any low end to even spin when it's raining now. I don't know if the torque management has anything to do with that now or not but it will never break traction.
Also according to the solus, revving and idling and driving down the rd it was at a constant 14.6:1
Only thing that seemed off was when I backed probed the 5v reference it was reading like .5 or so. But once the sensor is unplugged and still back probed it reads 4.9v where it should be. But if there was an issue shouldn't I receive a MAF code
Thanks all

 

What's the mileage on your truck? I wouldn't rule out plugged cats just because it sounds normal. Also have you tried a vacuum test? That might give you a better clue to what's going on. Any mods to the truck?

 

No I do not have any mods what so ever, and it has a 180k. I haven't done vacuum test just carb cleaner .

 

Clean your MAF lately? If not, get some electronic cleaner and spray this **** out of it.

It's a cheap, easy try.

 

Yes I did a couple months ago with CFC brand. An entire can.

 

Might be worthwhile to do a leakdown test, it will sometimes show things that a compression test wont. Clogged cats was my second thought.

 

Have you logged timing? I have seen the Knock sensors fill up with moisture and jack with the timing.

 

I haven't logged but I watched while driving it was around 25 idle, the hit around 30 speeding up and continued up from there. Nothing our of the ordinary though.

 

So I hooked a 10psi gauge with a rigged up 02 sensor and I'm getting roughly 6psi between exhaust manifold and cat? Sounds bad, is that alot? I assume it needs to be under 1 to 1.5?

 

That sounds pretty excessive to me, but ive never measured a stock one.