I'd say count your past blessings....it's back to reality. These trucks are like a bread box on the road...great fuel economy isn't one of their big achievements. I'd like to see 25 mpg on my truck also, but knowing its just not possible.
I've heard similar from others that installed those 3'' rc lift kits without changing to new a arms to correct the geometry back to stock. If you don't, you get vibrations at various speeds. Installing larger wheels isn't the wisest thing to do either. I've been told more than once to leave...
Most I ever saw with my 3.5 tacoma was almost 23 mpg, and that was coming back home from a 160 mile trip to Boston doing around 70 mph. It's always been around 21-22 mpg on roads here with a combination of city /highway. I still get 19 towing my 5'x10' utility trailer, and snowmobile...thats...