ISX mileage dropped suddenly

[ColdDiesel]

I am going by hand calculated fuel mileage. I bough the RV from a dealership and didn’t know the emissions had been deleted. It’s not legal for them to sell it like that, so they paid the bill to put it all back to stock. I had many reasons for doing this. Bad deletes can damage the motor. I plan to travel in the US a lot including California with it. Nobody will touch a deleted motor if you have problems in many places. You can’t trade it in to a dealer if it’s deleted, and it hurts the resale value as many people won’t touch one if it’s been messed with. I know that I would have never bought it if I had known it was deleted myself. I guess the last reason is that it’s just not legal period, and I am the kind of guy who will follow the rules even if I don’t agree with them. I don’t really agree with all this emissions crap myself, but I also am not going to break the law.
The reason I am confused by the sudden mileage drop is that 2.5mpg is a huge drop. I also see the EGR valve is open most the time on the highway and I thought it was usually closed. But I had no access to this info before the mileage drop so no data to compare it to. But I am driving the same speed as always, 65mph on mostly flat roads. So I wish I knew where to look for a problem, when I have no codes.