Same basic problems as putting solar on a car though...
- Cost of the panels vs amount of energy generated makes it a lousy trade off.
- You might be talking a lot more square feet of power generation, but the truck also requires a lot more energy to run.
if you put solar on the trailer and leave it out in the sun while the truck makes another run then you will want to add batteries to the trailer as well. That adds complexity, cost and weight. The only exception to this might be using batteries and solar on the trailer to provide refrigeration.
As a company looking to go electric they would be better off putting the solar on the roof of the building where it will not get damaged or dirty being driven around the countryside.
Couple that with a mega charger or two and batteries to create storage on site and you have cheaper more reliable power for the building as well as your fleet.