I have a 2004 Envoy, that gets 11/13 mpg, when we got it a couple years ago it got around 20mpg, it just keeps getting worse. I have replaced spark plugs, A/C pump, air filter, brakes, tires, Changed to full synthetic oil, both O2 sensors and even took out the cat I use fuel additives also. I have had it into 4 different mechanics, 2 GM and 2 back yard mechanics. No one can tell me what it wrong. not sure what to do next, short of replacing the engine or the whole SUV.

First of all the cat should be reinstalled. This can be a serious violation, plus it is under warranty for 7/70. Second only seeing the results of a full scan would help diagnose this, plus all the history, any tires replaced any history or previous cel codes? Change in route or driving habits? 4wd or 2wd? There is no quick answer here, only site diagnostics will help.

Are you using gasoline w/10% ethanol? have seen a slight decrease in mileage with this. Citgo, Valero.
Toyota posted a dealeship campain to stay away from these fuels.
Food for thought!

I seems all the gas in NYS has 10%.

I sure hope not. If they do it has to be published on the pump. I have'nt seen this on HESS or Kwick fill or GULF.
I hope these guys aren't pulling our leg.

I live in the Lk George area, I have not seen any pumps from NYC to Albany to upstate that are not ethanol marked, small tag, but it was there. BTW you may loose about 1 mpg using e10. If the pumps in Mich are not marked, which the feds require, than it is not e10.