It could also possibly be the quality of the diesel fuel.
I know in my car depending on where I get fuel and also the time of year reflects how many miles I can get for example:

Texaco diesel gives me about 298 miles to the tank{expensive cheap quality watered down fuel =) } , Citgo 330-345 m.p.t. (more in the winter)
exxon about 219-225 m.p.t., haven't tried Sunoco yet bet I get the best gasoline milage from them.

Other than that possibly your system needs a good purge. that would be the cheapest route to go and it would never hurt to give it a shot.

well good luck hope everything goes back to normal

Jeff