Absolutely rat snake. It's not all that large(15-20 ft? Really??). In the video, you can clearly see that the tree is actually only 4-5 inches across, so it's a nice big ratsnake... not a Macklots, or a retic or a prehistoric cave snake or a yeti finger.
The markings are there, just extremely dark, so it's probably about to shed too. Doing the math with the tree trunk being about 5 inches across(diameter), I'd guess it's about 7-8 ft at most(can't see the entire snake).
Next up... we need a new photo of another 40 ft gator.