This is an interesting idea. It has some major problems. Something similar is already being used: trucks hauling ore mined at higher elevations use regenerative braking to charge their battery coming downhill, which is enough to propel the empty truck up the hill for another load, with no need to charge the truck.
There are some disadvantages. One is the constant wear and tear on the roads and vehicles. Another is the requirement that drivers have to drive the electric trucks. And since mountainous regions typically have roads that are difficult to travel, there is greater danger that the trucks could get in an accident.
I would consider picohydropower or microhydropower before considering this truck hydropower.