Same problem with tissue under fingernails - very small quantity to begin with, exposed to the environment, foreign tissue decomposing simultaneously with victim tissue.
My opinion...best chance for foreign DNA would be from semen. Sperm cells are some of the toughest cells in the body - difficult to break down, long-lasting. If the body was still wearing the pants, the pants would have protected a portion of her body from animal activity. This would allow for the possible collection of vaginal/cervical swabs. If she was wearing pants, she may have been wearing underpants which could have semen on them from recent sexual contact. This semen could persist despite the conditions.
Not the same thing I know, but I'm still hoping there's a beer bottle from his car with his prints, her DNA, and a trace of a date rape drug that led to the first charge.