It has a floating ground which your panel sees as reversed polarity even though it really isn't. As Boomerang said, connecting the ground (green) and neutral (white) together will fix it. Installed generators and inverters do this automatically, but neutral and ground may never be bonded on the boat when on shore power. So if you connect them in the plug, that plug may only be used for the portable generator. Don't use your shore power cable for this. Another option is to make up a bonding plug which is just a three-wire plug with the white and green terminals connected but nothing on the black terminal. Plug this into the Honda's other outlet. That bonds the neutral and ground for both outlets. Or just ignore the RP light. Not ideal though, you could have a real reverse polarity and not know it.
