OK, you probably know Manon des Sources better than I do (esp. since I have to rely on subtitles!), but the role that the truck plays is that it brings water to the town after Manon found the source in the mountain and decided to block it, in the process cutting off Ugolin's water on the Romarins farm, as well as the town's. This was a deliberate action on her part to punish all of them for their collusion in hiding the spring which lead to her father's (Jean Cadoret/ de Florette) death so that the Soubreyans could get the farm (with the spring that they blocked on it) for their carnation-growing.
The whole water issue is central to the story, as is the delivery of water to the town; hence my uncertainty about the role of the truck (which really only appears stationary in one scene). I am really open to your suggestions here.