2009 Toyota Tacoma fuel milage

I purchased this truck new and loved every thing about it for the first 6 months, then on my way home from work one night approximately 6 months ago the truck stumbled and felt like it was going to stall out (I was driving up a hill, engine at operating temperature), this sensation lasted approximately 10 (ten) seconds and the engine seemed to smooth it's self out. The problem I'm having is that ever since this episode I've experienced a dramatic drop in the fuel milage, and when driving any kind of an incline the truck generates a rotten egg, flooded, gassy smell. Since that day I have also lost over 150 (100 miles) kilometers per tank fill up. The dealer where I purchased the truck in Kentville Nova Scotia doesn't seem to want to help me with this problem and tells me there is nothing wrong with the vehicle, yet the truck has gone from 28 mpg, to 22 mpg, and it doesn't matter what kind of driving I do. Winter, summer. Town. Highway, the best I can get is 22 MPG. I did get a dealer 60 miles from the dealer from which I purchased the truck to scan the engine and they suggested that the O2 sensors were on the low end of the operational value and installed new ones under the warranty. This did not help with the fuel milage, and did not help with the smells that the engine has been generating, it did nothing to help with the fuel milage. I know there is something wrong with my truck, please help me find the issue so that I can get it fixed before the warranty expires.