Do you know which circuit breaker they are on? If they share a breaker with other outlets, it's a matter of figuring out which one.
Are you good with electrical work. This next part isn't for a novice. If you're not comfortable working in the circuit breaker panel, stop now!
Most likely what is open is only the hot wire (black). You need a long jumper wire. It needs to reach from the outlet to the breaker panel. With the shore power, generator, and all inverters off, do the following. Attach the jumper wire to the ground wire (green) of the outlet. With a continuity tester or VOM, check for continuity between the ground buss for the panel and your jumper wire. If you have continuity, then it's a matter of removing ground wires off the ground buss one at a time and checking for continuity between that wire and the jumper. While this may sound like a lot of work, there should be a number of circuits that aren't likely candidates such as air conditioning or water heater.
Once you figure out the shared circuit, it shouldn't be to difficult to sleuth out the failure point. While you could use the same process with the neutral (white wire), grounds always end up going to the ground buss.
Ted