2005 Yukon MPG goes down about 2 months after tune up

dfcmxp0

Hope someone may have some info on this. I bought a used Yukon in May of 13 from a dealer who claimed they had just done a tune up. My Hwy miles after purchase averaged 20-23 mpg, after about 2 months I noticed the average went way down to about 15-16 mpg hwy. In December I decided to change the plugs and wires myself and my Hwy miles returned to 20-23mpg, this was great until it slowly started going back down to the 15-16 again. When I examined the old plugs they had a medium grey color, base of plug was pretty dark. Burning rich maybe? If so, what would be the best way to correct

Any ideas? Thanks in advance!