I agree with everything you're saying. America's unique position--geographic location, physical size, and wealth of natural resources--means it's far better able to weather any financial storms. That said, America would absolutely be isolated. It's freedom of maneuver (social and trade...not military) in occupied territories may be significantly impacted and, just like Germany and Japan couldn't impact America, it would be very hard for America to impact the autocratic power blocs.
America could continue to turn inwards, and it possibly could survive on just its internal economy. However, all the while, Japan and Germany are growing their economies, milking the lands that they occupy. Ultimately, either Nazi Germany or the USSR would prevail after Barbarossa gets launched in 1941....so would we see 3 major powers in the world: the US, Japan, and the USSR, the latter occupying Europe? Having America surrounded by non-democratic nations, with no strong Allies, would certainly cramp America's style, particularly if the autocratic powers attempted to gang up on the US.
The US was able to become a superpower in part because it had strong democratic Allies around the world, primarily the UK, Canada, Australia and New Zealand, who shouldered some of the load, collaborated on technology and intelligence, and who were willing to support international trade and a shared set of values. Take those Allies out of the picture and America would lack a lot of the leverage it was able to exercise during and after WW2. America wouldn't be eliminated but its reach and power would be far more constrained than was the historical case.