are you saying Germany has taken over GB and all of Europe, or is eastern Europe still independent?
If they do not have complete control of GB then the US has a place to attempt to stop the ships and transports going across the Atlantic.
One problem with the scenario Germany doesn't have the natural resources, and space to compete with the food and manufacturing capabilities on the USA. Over the long term I don't think they'd be able to control, occupy, or stop the US from growing, building military equipment, and providing for our military.
Add in the amount of civilians who would fight back all over because of our right to bear arms the Germans would be pushed back. just think of down south and in the rural midwest areas. all those hunters, kids even that could shoot as well as a trained soldier. French Resistance? Imagine what the "American Resistance" would really be like.