fuse panel

When adding accessories to an rv and you need a switched ignition source can you use an empty slot in the fuse panel and install a fuse and use that fuse to power the unit?I have been using the radio fuse for multiple units and would like to seperate the wires this way.Also when testing the circult with a multimeter what would i set in on?12 volt,ac or what?Or would it work better to use a test light?