No worries. Volumes and weights are separate, mainly because the game is designed around volumes and I'm trying to write this version more how it would be if units were added to the base game. Since tons is a unit of weight/mass, you'd have to have weights toggled to see tons by default for bales. However, when you switch to a combine, it would then be in weight mode as well until you toggled the display back to volumes.
Anyway, most combines should show bushels by default without any configuration. It is set that way internally. To show tons as your default weight unit for the solid group, it would have to be the first weight unit in the <units> list of the xml. I believe that is also the internally registered default for version 0.0.0.5.
The above would make cubic yard be the only volume unit for the solid group. Ton would be the default weight unit, but you could switch to pounds if you wanted to. You could also leave pound out of that list, and you'd just see cubic yard or ton depending on if you had volume or weight toggled.
I just chose cubic yard for the volume for this example because that is the default internally registered default. You don't HAVE to make it that. The simple way to remember it is, the order of that list is the order you will see the units. Even if cubic yard was at the bottom of that list, though, it would still be the first and only VOLUME unit, so it would show first. Since there are two weight units in that example, the order is ton > pound because ton is the first WEIGHT unit listed and pound is the second.