On most RVs, there isn’t a nice wiring diagram with fusing, grounding and splice locations, wire colors, and printed circuit identifiers like we expect on a car. When something isn’t working, it’s often not obvious what circuit it’s on; when a fuse blows, we often don’t know what’s on the circuit that might have caused it.
It’s also pretty common for fuses to be oversized, not so much for the wiring, but such that if a problem were to occur, a fuse isn’t going to be very quick to blow. It’s not easy to work with a standard multimeter to connect inline, and measure the current with the load turned on.
Here’s what I use:
This is one of my favorite automotive electrical diagnostic tools beyond a multimeter. It’s a really simple current meter that plugs in where the fuse goes, and the fuse plugs in to the side of the connector. I can see that my water pump peaks at just under 5A, and the furnace at 10A. Same thing for lighting circuits, the refrigerator, etc. This also allows you to put together an estimate of what you need in terms of batteries, solar, generator runtime, etc. if you were to run something for a particular amount of time.
It’s not quite as sophisticated as the Kill-A-Watt meters used for 110V power, but does it’s job quite well.