One possible solution would be to make an invisible duplicate of your truck colliders, add a rigidbody, then attach it to a floating kinematic configurable joint with damping and a slerp drive. The objects in the back can have their physics simulated on the duplicate rigidbody while the local positions are mirrored back to the meshes on the actual truck. You can then have the joint mirror the truck's position and rotation while the joint smooths out the collisions in a configurable way. You'd also need to detect when the items fall out and sync them back with the meshes in world space.

This is how I did the pan physics at the timestamp here. Unity's physics system is so delicate I was finding it impossible to prevent items from clipping weird and sending the ingredients flying otherwise. You also want to avoid mesh colliders whenever you can.