The outlets are daisy-chained. You either have a loose connection (especially if the outlet is installed using the push-on connection) at an upstream outlet or you have aluminum wire and a PO has installed a copper-only outlet fixture. Either way, this is a fire hazard, so get on it.
Disconnect your shore power, take the outlet covers off, then remove each outlet one at time and check screws for tightness. If the wires are pushed into the back of the outlet, I recommend taking them out and using the screw terminals. Removing the wire from the push-in connection requires a very narrow flat blade into the small hole next to where the wire goes in.
If you're not familiar with outlets, the black wire goes on the brass colored screw.