I never used to think that a multimeter was a necessity in your rig. I figured that I would just work on it when I got home, where the multimeter is.
Recently, I needed a multimeter with a battery issue, and didn't have one, and no one that I was camping with did either.
Needless to say, I now have one in the tool drawer. Never used it in our unit, but had to let the son-in-law borrow it recently.