Turn the Multiplus off, and then check the ac input and output connections, inside the outlets, at the GFCI breaker and anywhere else the 120V ac goes to see if you can find a wired connection between ground and neutral conductors. Often this is installed in the distribution box / breaker box / consumer unit but you do not have one. This link wire is not required as the Multiplus creates this link when you are not connected to shore power. This is described in the manual download from Multiplus 2000 120V manual.
You will need to remove the ground neutral bridging wire AND check your Multiplus settings to make sure that the ground relay remains enabled as per default setting. You really need to be absolutely sure that you understand what is required and the safety aspects here.