3406E fuel mileage drop

Looking for some help from some of you guys with pre-emission cat engines and midroof trucks. Recently quit pulling stepdeck and started pulling dry van. Not sure if its a problem with truck or the trailer dragging air.. My fuel mileage has dropped about 2 mpg lately. I was around 6.3mpg with step deck. I have a 2000 freightliner classic with the peterbilt style midroof sleeper 500hp 2ws cat, 18 spd, 3.55, 22.5 super singles. Engine runs great and good power, tune up done 70,000mi ago. I was hoping to be at 5.5mpg or higher with a van and this combo. Is this normal for a midroof truck with cat? Only work done lately was had the transmission rebuilt. Everything else is kept up very well on it. Maybe something happenedto like an Injector leaking? Or plugged muffler possibly?

 

Alright I found my tires were all about 10psi low on truck and trailer so took care of that. Adjusted and checked all brakes. Tested CAC was ok. I still have my headache rack on so my gap between trailer and cab extender is 32". I picked up a used whale tail so ill put that on in a week and pull headache rack off... maybe that will make a big difference? As far as the ECM its a true 500 and hasnt been touched since new in 1999, i do all work myself besides oil changes. I will need rearends done soon so thought about switching to a 3.21 gear ratio instead of the 3.55 which im running now 65mph at 1500rpm. Has anyone done that and had good luck?