Truman won because he was so unpopular that he was forced to aggressively campaign, and his opponent, knowing how unpopular Truman was was content to resurrect a 19th century style “front porch” style campaign where he did and said little and just waited to be crowned with his party underlings doing a bit of ground work.
Against FDR, Dewey would have come out a lot stronger, and FDR unlike Truman was physically incapable at that point of giving 10 speeches a day or riding over 22,000 miles of train track hitting basically every corner of the country.
I don’t think FDR’s numbers would have dropped as far as Truman’s, but to imagine Americans engage in Presidential-worship such that we’d have ignored the economic problems of the mid-to-late 40s and just happily re-elect FDR with 80% of the vote is ludicrous. Winston Churchill was truly the FDR of the UK, and actually lead a people that faced a hell of a lot more serious risk from the Axis than America did, and they booted him out because of all the problems in the country after the war was over.
Economic crisis, three-way split in the party, and an un-precedented and frankly worrisome fifth term all would combine to hurt FDR’s chances. There was even talk at some point that some of the power brokers in the Democratic party were planning to work against FDR if he had ran again in '48, just to open the way back up for other Democrats to run for office.
Roosevelt was essentially alone until Pearl Harbor as an interventionist. Yeah, the Henry Cabot Lodge camp (basically following the ideology of Lodge who advocated for us to not join the League of Nations and etc after WWI) of the GOP was the biggest bulwark of isolationism but interventionism had virtually no support in either house of congress with either party until Pearl Harbor. After Pearl Harbor it had overwhelming support in both houses from both parties.
You are assuming the FDR could not have turned back to domestic affairs after the war over. I think he would have done that and done a better job than Truman did. I’m not saying it would be easy, but if FDR did run again he would have been a formidable campaigner. But I do think his status as the man who won the war would have carried him much further than you are thinking. The evidence of this is the election of Eisenhower after Truman. His status as a war general was his only asset and he was easily elected based on that. It was a different time, and our embroilment in Korea was certainly an advantage for him. But the threat of the Soviet Union was rising and FDR’s influence with Stalin could have made that just as important a point. However as I have already noted, FDR would have to have been in much better health to have run again at all. Given that I think he could have dealt with domestic and foriegn affairs skillfully and successfully.