For me, Fluke 73 series II multimeter. There are better meters available (particularly for current measurement), but this is what I have and it serves very well indeed, and has for over 20 years.
For 120V AC circuits, a non-contact voltage detector is very handy.
I don't keep these in my RV because I use them in my house at least as much. In the RV I have an inexpensive analog multimeter, maybe a half a step up from the free HF meters, which isn't as nice to use or as accurate as the Fluke but is sufficient for troubleshooting RV electrical problems in a pinch (and for spot-checking campground voltages).
Probably the best tool is a keen mind and logical thinking. Sometimes I bring mine along, but sometimes I forget and leave it behind somewhere.
