I have experienced a somewhat similiar fall in MPG with my HH.
I even took it back to the dealer who drove it with a computer attached and gave me the printout which indicated 35 mpg hiway and 55 mpg city ???
NOT what I had experienced...however...I took it upon myself to check the tire pressures and found my left front was @ 22 psi when it should have been @ 32psi...
Since I just recently purchased the vehicle, thats the way it came from the dealership @#$%^&*&...
After filling all to 32 psi my mileage improved significantly, as my latest manual calculation showed a 26.29 mpg vs the previous 22.756....
Some on the HH board have suggested a 4-5 psi increase above the recommended 32 psi for better mileage during the Summer months.
Come Winter I would deflate to the recommended psi for better handling.