HORRIBLE fuel mileage

[Redland1]

My mpg is down to 4mpg and the last fill up was 4.33mpg.Engine is an 04’ ISX 15 535 hp in an 03’ international 9900i.
3.90 gear ratio and 13 speed.Driving habits have not changed and I’m still hauling the same commodities with the same trailer only difference is that I’m loading 6k lighter bringing my gross to 96k.The only thing that has changed or been done to the truck is a demandate,EGR block plate,and the plate that goes where the EGR entered the intake.I did have an exhaust leak that I could feel around the EGR block off plate when I first installed it and I tightened it down even further and I can no longer feel it at idle but don’t know what it’s doing going down the road.What do I need to check so that I can get my fuel mileage back in check.Prior to the demandate I was getting 5.5-6mpg at 102,000lbs gross.Demandate was done with stock 535/1850 torque..