Wow -- that seems like a hard problem to set up for Solver, or to model in code.

Start at whatever location the trailer camped for the night.

Pick up some fraction of the deliverables for other locations, up to the available limit. Drive to the next location; drop off its deliverables and fill the truck. Rinse & repeat.

In each step, you need to optimize both the pickups and the order of locations visited.

Ignoring that you have a second trailer, it takes several circuits to complete the deliverables.

You might take this the the operations research department of a good college.