That would be my guess too. I've always heard 40-60% as well, and when I was with my ex who had a RTB, we always kept her humidity around 50% and she was fine.
I know they definitely do seem to enjoy their water dishes, although 2 days does seem a bit excessive. I think the longest I ever saw my ex's RTB in her water was around a day, but maybe it varies by individual, like temperament. I'd say as long as he has 2 good hides, it was probably just the temp spike.