@General:
And here’s another: when those trucks return to the off-board area, can they take something with them? If there are three supply tokens and an arty, can the truck take those off-board to be brought back on immediately, if desired?
Once units are on the board they are on the board except for trucks and aircraft. You can’t pick up truckable units and take them back to the reinforcement chart. Likewise tanks can’t move off board on their own.
The rules are vague about truck capacity. I understand it to be six units: two arty items. Or could a truck haul six artillery?
Per page 11 of the rulebook there is no restriction on how many artillery a truck can handle. Any combo of supplies, infantry and artillery not greater than six.