I personally have not had this issue i have repaired 2 offroads up driven them. The issue i have with them is they are not persistent if left unattended for 1 - 3 hours they despawn on a persistence enabled server. Have you tryed v3s's ect on another server if yes the only thing i can suggest trying is verifying game files.

Ok welll i have noticed there is a bug where if the truck is too close to a tree, IE the tree is covering the truck you no long get the option to interact with it. Is the truck under a tree of some form?

To enter the trucks you need to have open hands. have you tryed that?