If you don't know why you need it or how to use it, pickup a multimeter from Harbor Freight for under $10. Then when someone is helping you and ask if you have one hand it to them to show you how to use it.
Only partly kidding, for most RV uses you don't need an expensive meter to do the job. If you have never used one before you can burn them out if used wrong and it is better to burn out a cheap one.
I have several good ones scattered around the shop but keep a cheap one in the RV for that once every few years I need one.
