IMO...
Meter reader called in tip in august, nothing came of it...his hinky meter was going off so he went back the next day and maybe got the same deputy..again not feeling secure with the outcome, he goes again and calls...the deputy acts as if he'll really look into it or threatens the meter reader of something if he keeps calling/harassing. I believe the deputy will end up being the same guy who was fired for having relations with casey. The meter reader sits on it for a while, then goes back but it's all flooded...he waits until the flooding goes down (c'mon, just a coincendence that as soon as the water is down he' back?? he was waiting for it!). This time, tired of just claiming a trashbag found, he decides to kick it or shuffle it with his foot and out comes a skull - he calls his superior and LE, and this time with a much more serious find that he knows they'll look into.
I believe he is totally credible. Just a really concerned citizen. What other reason would he have? Either the clues will free her or incriminate her more.