2001 GMC T7500 3126 Cat squealing alternator/belt

2001 GMC T7500 (tilt cab) 3126 cat 210hp. mobile home toter.
I lost serpentien belt 1 week ago on my way home, started with a high pitch hummm.
stopped to check, no evidence of worn belt. so got back on the rd. about 30 miles later hummin would come and go.then lost serpentine belt, limped it to rest area, shut down and have since replaced all of the following.(pain in the but to lift cab and service.)
belt, tensioner, ac compressor, alternator, fan clutch bearing.
keep getting high pitch squeal.
tried 2 different brand new delco remmy alternators.
1 remaned Wilson alternator
1 used alternator.
4 different belts( heavy duty grade belts are too loose,)
tried switching pulley on alternator.
tried putting old tensioner on.
2 new batteries (delco)
it now only squeals when putting load or strain on alternator as in driving down the rd.
at idle has stopped squealing.
cant seem to get nut torqued to 70/80 lbs. without stripping inside of aluminum shaft.
have tried an impact also to get tighter.

I am no good at wrenching. however seasoned mechanic has looked at this, and half of the replaced items are due to trial and error.
still no luck.

I am at this point leaning towards needing an OEM alternator which comes with the pulley, and I can only assume the pully will be already torqued to specs.
I am feeling like the motor is just putting too much strain on pulley on alternator and wont tighten down enough.
please forgive my ignorance, I am going broke in a hurry and stressed out, as if I cant run the truck I cant earn.

 

already replaced ac compressor brand new $450
not able to turn alternator with belt on by hand.
only 2 options in belts for this truck, have tried both belts, and 2 different belts for each option

 

this truck only runs ac compressor, alternator and tensioner on serpentine which runs off main engine pulley,
water pump runs on separate pulley and belt and noise is not present when serpentine is not on.
I did spin each device by hand, ( only tensioner sounds like bearing are bad and only evident when off truck and holding next to your ear.)
I believe the tensioner went bad which caused belt to let go.(not seized but bearings do sound loose)
I chose to do alternator as I was in there and trying to be diligent about preventative.
a road service tech is only as good as his experience. I am not a complete dumb ###.
as I said a seasoned mechanic is also having trouble diagnosing this.
by the way an OEM alternator is the only alternator that comes with pulley for this truck, hence the reason I am leaning toward getting one. (this application is considered external for cat, therefore it is an AC/DELCO unit)
just looking for other NON EVIDENT ideas which I have mentioned above. before I spend more $ on something that may or may not fix.
I was really hoping for a seasoned well versed mechanic to respond, Really don't care for the driver responses or the part time wrench responses.
a cat experienced mechanic response would be great,

 

are you a mechanic?
just asking?