The window for this is after 4/17 when the USA declares war, and spring 18 when the US buildup in Europe is well underway. Basically a combination of a more widespread French mutiny where the troops actually desert/leave the trenches (OTL they merely refused any offensive but stayed on the defensive), and then the Germans manage to move lots of troops west after the Russian collapse. This combines with a German offensive that splits the seam between the French and British, and results in a French collapse. The British can fort up along the channel as no WWII level airpower or tanks, so being thrown in to the sea not happening.
Here the USA has some significant casualties as not quite ready units are rushed forward to try and plug holes. When the French throw in the sponge, its all over. I doubt there is much the Germans could expect to get from the USA. By 1918 the German fleet has been reduced, and in any case is not designed to operate long distance unlike the USN (and RN). The scenario of the Germans using Mexico (or similar) as a base to attack the USA is possible, but that would take time to get the Mexicans on board and get men and equipment over there. Seeing the French having to give Caribbean islands or French Guiana as part of a war reparation as possible.
IMHO Germany even after "winning" has been bled white, been through "starvation winter", and financially strained. The last thing they need is to get in to a hassle with the USA about peace terms. I would expect that the Germans and the USA will come to terms quickly, the Germans demanding return of assets seized (companies, patents, etc) and little if anything else. Postwar I think US isolationism and disillusion with European involvement will be even more than OTL. The USA spent treasure, loans never to be repaid, dead and wounded to bail out the Europeans - who folded anyways. As part of this however, you might see more support for a strong navy, and a somewhat larger peacetime army and proper reserve/national guard system (mobilization for the Mexican affair in 1916 was a mess) for continental defense - especially if Germany makes inroads in Central/South America.
Basically once the USA has large numbers of adequately trained troops and equipment in France, the Germans are toast, even with Russia out of the war. OTL the MICHAEL offensive was a manifestation of this but was too little and too late.