ISX mileage dropped suddenly

[ColdDiesel]

Hi,
I have a 2014 ISX motor in my RV. When I bought it, it had been deleted and I was not aware of it. So I had the emissions systems all repaired put back on it. So now it’s fully stock in every way.
I am getting no codes at all. But my mileage was around 7.5mpg in an RV weighing around 50K pounds initially. But now that’s dropped to around 5mpg for apparent reason at all. Is there anything anybody could think is the problem? I did get OTR diagnostics so I can monitor more things on it, but didn’t have it delivered before my mileage dropped by 2.5pmg. Any advice would be appreciated.