Had the same thing on my Ryobi inverter generator. I cut the 110 to the house and connected the generator and suddenly the polarity was correct. When I connected the generator to the house I connected it to the house ground. The generator detected this and adjusted. This is the floating neutral feature. Try running a ground stake connected to the ground screw and see if the generator doesn't adjust and correct the issue. Generally AC devices with run fine no matter the polarity. In a house it can generate safety issues so it does matter. In your RV it is probably fine.