While you may not see a "Nazi" America, there was certainly a good number of Americans who admired Hitler's methods and results prior to WWII (getting rid of Communists, reining in labor unions, putting Jews in their place). In a scenario where Germany "wins" - Dunkirk fails, Churchill out & replaced by Halifax and Russia standing alone against Germany (which IMHO could result in the Germans taking what they want in the east) an America drifting towards fascism is not unrealistic. {read Sinclair Lewis' "It Can't Happen Here"} An America with blacks everywhere disenfranchised and national Jim Crow to the max. The quotas and restrictions on Jews in education, housing, employment widened. A supreme Court that allows a good deal of flexibility on free speech suppression even without a constitutional amendment.
In that sort of Germany wins scenario, I expect you'd either see a USA arming to the teeth and building what alliances it could, and do9ing everything and anything to keep the Nazis out of the western hemisphere, or an America that goes well down the road to fascism if not all the way. An America that has the attitudes and politics of 1938 won't be possible in a Nazis win scenario.
If the Germans are allowed to "occupy" Canada, or build bases in Mexico then an "invasion" is possible, but the USA allowing either of these is as unlikely as "Fall Adler" with an amphibious landing on Long Island.