2009 columbia detroit s60 14L bad mpg

I have a 2009 freightliner Columbia with detroit s60 14L with dpf my average fuel mileage when first bought the truck may of 2014 @455hp i was getting 4 to 4.4 mpg so i disabled my egr and dpf mikeage was still crappy i raised up horsepower to 515 and torque at 1650 changed the intercooler with the pipes that got to intake im assuming guy did ecm tune i was getting after 5.4 to 6 mpg at 70 mph average 75k ever since winter started im back to 4.5 mpg any ideas what to do?

 

I also had a injector changed in the summer put the mechanics didnt changed the gasket i can see oil moist i also asked for a tune up but im not sure if they did it cause i was charged 1000 for changing injector 1 and an oil change also when we put the computer on it brings up an inactive code of the actuator we test it it comes back ok you guys think that could be the issue? Also sometimes truck doesnt feel as strong

 

I haul flatbed yes average of 75k is my weight so ur saying the blend makes a difference but 1 mpg drop its a lot

 

Wheres that place located at?

 

I put only #2 diesel i sleep with the truck off cause i have the webasto heater and im getting that dam fuel mileage i had an injector changed i told them to do an overhead but honestly i dont think they did price was low you think freightliner are good on overheads or should i just look for a good mechanic to do one?

 

Thats my problem i gotta spend most of my time with the pedal to the floor to pick up speed what i dont get its that if i haul a 45k load compared to 35k truck responds thr same on the hills and fuel mileage is the same truck really pissing me off and my truck is a 13speed