Diesel fuel starts to cloud up at about 20 degrees. So you need to have projected extended below freezing temps for there to be problems which require fuel treatment at the pump. I'm not from Texas but I'd be surprised if they do much of anything to the straight #2 in San Antonio. And if they do, they can treat with chemicals instead of adding #1 diesel. You won't know unless you ask - and expect a blank stare unless you are at a truck stop. So I don't think that's the problem here. Even if they treat with #1, I'd guess the percentage is small.
Did the MPG drop right after the air cleaner change ?
Has it thrown any codes ?
Diesels are un-throttled engines. They flow a huge amount of air. I would go back to the OEM filter system and see if MPG goes back up. When something changes, I always look back at the last thing I did - that's usually what caused the change.
If you go back to a paper filter, get the Ford one. The after market versions can collapse due to the large airflow.
I get about 14-15 mpg around town from my 2013 F350 4x4 dually. I get over 20 mpg at 55-60 mpg and about 19 mpg at 65. All non-towing.
