It does not sound so bad, 5mpg but I am talking about going from 26 mpg down to 15 and makes the overall average 15 so it is dropping 11 mpg. It won't be driven much until spring. It will get sorted out.
26 MPG? Excuse me for being skeptical, but are you sure that it wasn't 26 KPH? I do recall your earlier posts of crazy good mileage, and I was surprised yet happy for you.
Now I am wondering if there may be something up with your prior calculation of gas mileage, and that your current calculations are normal for your truck.
Carl