It's very easy, but yes, you're right, you should have asked it first.
You simply duplicate the Background layer, the turn on grids and Ctrl T to get the transform handles, and rotate until the horizon is straight.
Then add a new blank layer and clone in the sides of the truck a bit:
Then group those two layers (the rotated layer and the clone layer) and again, add a black mask to the group. Paint white to reveal the new straight horizon: