As stated before, either a bad ground or primary wire. With your VM plugged into the outlet, run a wire from a good ground (negative battery terminal is ideal) and touch the outside of the outlet. If your voltage increases, you have a ground issue. If you still read low voltage than you have probably have trouble in the primary wiring circuit. Start with the fuse block and read the voltage on either side of the fuse. If OK, then look at the wire going to the outlet. Is it brittle? Hard? Discolored? This may indicate an overload at some point in time and the cure is to splice in another wire where the damage area is, making sure you get back to bright and shiny copper. A nicked wire or pinched wire could cause the issue as well. Sometimes, it is easier to cut the wire at both ends and run a new wire from the fuse block to the outlet. Good luck.