I don't know about a 6.0, but my neighbor routinely tows 25,000-30,000 on a gooseneck with a single cab 7.3L F-450 for his business. I know he has grossed around 40,000 at the scales before. I know 10k is a lot of weight, but I don't see why you couldn't tow 10k more if you weren't in a hurry and took it easy. If I were to bet on it, I would guess he has grossed over 35k at one time or another.
Regardless, it isn't something I would recommend very often or for very long/fast. I have no doubt it could be done if loaded just right.