Wow this is puzzling for sure.
Your 285/50/20 tires measure about 31.20" tall. My 305/40/22 tires measure 31.60 so yours are a little bit shorter overall height (which should help your cause if anything). If your keys are completely de-cranked, that means you are missing a good 1-1.7" of drop somewhere which still kinda points to the possibility of someone installing lift/leveling keys before you owned it.
I would maybe try to round up some stock keys from a salvage yard Tahoe and install those just to be sure you are working with stock keys. They are fairly easy to swap out in less than an hour. The other option is to purchase some drop keys. The only downside to that is, if you unknowingly have lift/leveling keys installed now, you will be severely dropped in the other direction with drop keys, even with them fully cranked up.
Mike