A wiring diagram shows how the wires are connected devices (pin or connection numbers), what their color code is, what pins they connect to, etc. It's extremely useful for trouble shooting. I'm an electrical engineer and the first thing I go for if there is an electrical problem is the wiring diagram. The circuitry on something like an RTV isn't rocket science. If there is an issue, the problem is what is connected where - for that the wiring diagram is your friend.
My RTV900 service manual has a very useful wiring diagram on page 8-M1. In addition to that diagram, there is a system specific diagram for the various electrical systems in the electrical section of the manual. Between the various diagrams in the electrical section I've never had an electrical problem I couldn't solve on the RTV, or our ZD21 (using the ZD21 service manual).
My issue with the RTV service manual is with the mechanical part. While has nice tables of torque values and specifications it's really sparse on take down and reassembly instructions compared to the automotive manuals I'm used to working with. The expanded parts drawings on the Kubota web site are invaluable for figuring out what has to be done. And they are free.
Fitch