id invest in a scan tool of some kind (id recommend scangauge II from tone) for 160 u get the ability to read your own codes and lots of other engine vitals
as for the mpg i dunno what to tell you.. sounds like its getting progressively worse? which would mean somethin clogged and getting more clogged.. fuel filter.. air filter.. egr.. for now id probably remove the roof rack and fairing and window deflectors if youve got them as they are just adding drag.. and sounds like until you figure the problem out u need as much aerodynamics as you can get