Diesel fuel used to be cheaper than gasoline. At that time it was actually possible to make a diesel MH cheaper to run over a five or six year period than a gas MH enough to pay for the price difference between the two units.
Diesel has always gone up in price in the fall and winter because of the increased demand for heating oil in the northern/ northeast US. Heating oil and diesel are very similar so demand exceeds production capacity at times. The colder the winter, the bigger the supply gap.
The introduction of ULSD to combat some of the massive pollution issues with old style diesel fuel has raised the cost. It also raises some of the cost due to changes needed in the infrastructure to store and deliver ULSD.
There have also been conversions of some refineries which has resulted in more supply of gasoline and lower supply of diesel.