Eleanor Roosevelt was very popular, but I can't see her winning the Democratic nomination in an era of party bosses.
I imagine open sexism would hand Dewey a comfortable victory.
Eleanor was loved by Americans, but the South would revolt in bigger numbers.
Would it matter though, since Truman won without them? Also, the West may be more open to a woman candidate so she could still win those states, and she may have been more likely to win New York. Likely to be fewer defections to Wallace, and without those Truman would have won it as it was.
Possible winning map for her, even losing CA, IA, OH, and VA:
I don't think GA or most of the other Southern states that didn't defect were ready to flip yet, and the Roosevets' personal connection to GA (Warm Springs) might have helped a bit too. Even if Eleanor lost like 10 points compared to Truman in some of these states, to Thurmond or Dewey, she still would have won them.
She could not, however, survive losing Illinois in addition to CA and OH, even while winning NY.