A full tank avoids condensation. A drastic drop in temperature within the fuel tank can cause water droplets to form. This water can then mix with the diesel, which isn't good. Too much water ...

Diesel engines face a lot of challenges with cold starts. These engines have a higher compression of 14:1 to 25:1 as opposed to the lower 8:1 to 12:1 compressions in gasoline engines.

Temps down to -20F every winter. Most winter days stay below 32F. Yes, garage storage of the buses with moderate climate control is most helpful to precondition the battery and passenger space.