Hi,
Is the 85f the hottest part of the tub? You could try expanding the thermal gradient to give him access to a hot hide temp of around 90-94f.
**edit**
Re-read the bit about the cornsnake - I would put up a thread in the corn snake forum. Maybe having a temp gradient might help rather than just a set ambient?
**end edit**
How where are you measuring these temps by the way?
dr del