Tri-Polar World
Assuming I've understood the scenario correctly, there would in all probablility be a tri-polar world by the late 1940s.
I would at this point add a further caveat/assumption - either NO atomic bomb or that all three major powers have the Bomb.
So much would depend on whether the Axis powers have been expansionist or whether they have remained within the borders, if you like. One area of huge interest would be the Middle East. In some scenarios, a victorious Rommel conquers Cairo, Jerusalem and sweeps up to Baku. However, if Britain yields to a negotiated peace, the North African theatre probably wouldn't happen apart from some limited Italian efforts.
Presumably a victorious Italy/Germany would claim Malta but the British might be left with Palestine, Cyprus, Transjordan, Mesopotamia etc.
Where I'm going with this is that IF the Germans don't directly control Middle East oil, it will be a potential flashpoint for all three superpowers in the post-war world. Yes, they will all have their own sources of oil but Saudi oil would be the key to world dominance.
Back to the US and I'm not a supporter of "Fortress America" as a theory. Indeed, I think that while a modus vivendi would have to evolve between America and Germany, there would be clearer tension (initially) with Japan in the Pacific.
Would we see German efforts to undermine pro-Washington Governments in Latin America ? Could this reality's "Cuban Missile Crisis" be the election of a pro-German Government in say Columbia and an invitation to host German missiles ? Possible and plausible.
Without the impetus of war, the American economy would be behind the OTL version but would still be strong - there would, I think, be an initial shift of emphasis to the Pacific.
With no Vietnam War in this reality, that doesn't mean no American forces in combat. Would, for example, American troops intervene in Africa to prop up anti-German post-colonial Governments ? Where would South Africa be, pro-German or pro-American ?
And then we're back to the Middle East - I would imagine the Imperial remanants looking to Washington for support rather than Berlin but would Berlin back Turkey for example ?
What if, for example, a pro-German Mesopotamia invaded Kuwait in 1990 ?
America would be as involved in world affairs as in OTL but in a different way. Would the likes of JFK and George Bush Senior, without their wartime experiences, have still risen to the political summit ?
Assuming Roosevelt wins in 1940, would he still prevail in 1944 ?