Update. Skull appears to be back to normal. He's loosely curled on top of his hide on the warm side of the tank and appears to be sound asleep. He's been calm and still all day, only occasionally shifting around a bit. Today, we found a used 20 gallon tank that was last used at a local pet store for a corn snake. It's got a place for a UTH. We've ordered a good thermostat. In the meantime, we went to another pet store here and got another hide, some aspen bedding to pad his hides (plus some newspaper) and a driftwood branch to place in the new tank. We also got a cheaper thermostat to use until the good one gets here. Hubby is already working on cleaning and sterilizing the new tank. Then he'll set it up with all the stuff in it and make sure then temps are right, then we'll move Skull.

I'm not sure precisely when we'll do that as Skull's next feeding is on Monday and we know we need to leave him be to digest. We're also going on a brief vacation at the end of next week so we may not move him until we get back.

Again, my thanks to every single person who has contributed to this thread. Y'all probably saved my little guy's life as hubby and I were so distraught we didn't think to do the obvious which was to check the temps. Now we know.