There are many things that can go wrong over time. Corrosion is a big problem and can cause loss of contact within the box so listen carefully for "buzzing" or sparking when you plug in. Someone may have thought there was a problem when there wasn't and changed the wiring in the box, making it wrong or one box in a series may have been wired wrong and fixing it would have made all the rest of the boxes downstream from it wrong.
We have all probably run into the "hot skin" problem where we get a light shock when we touch out RV. This can be solved by using our battery jumper cables to connect the chassis to a metal water pipe or conduit or a re-bar that we pound into the ground. Pour water onto the ground before you pound your re-bar in and carry more than one because you will probably not be able to pull it back out.
With voltage, I would rather see a number on a meter than simply a green light. The bottom line is, it is really up to us to check things out before we plug in.
