Yeah, the scene with Carl retrieving the zombie killing "kit" is all messed up. Why is that dead body in the truck? the door was closed, did whatever zombie attacked it close the doors? If bitten, why did it not reanimate? If not bitten, why was it dead in the truck? The same could be asked for almost every body found in a closed space.
As for my earlier spoiler post, that revelation takes place much later in the comics, so there may be a time issue there.