The basking spot needs to be higher. 130-140 degrees. You need a temp gun asap. Your temps are most likely WAY off since your just using the cheap junk from petsmart or petco. Those thermometers and hygrometers dont work, especially for the surface temps that you need for your monitor. And Im sure the temps are off with those. Im betting the ambient is about 10 degrees off in either way.

No offense, but going from a corn and king to a water monitor is a HUGE leap. Even monitor keepers wont keep niles because they are very demanding. As an adult the enclosure will be huge and he'll eat you out of house and home. That 5x4 you have now wont last very long. He'll be growing like crazy. As of now, I wouldnt ever dream of getting a water monitor and dont think I ever will. This is because I dont think I could really properly care for such a large monitor.

You say this hasn't happened to you before. This is because you've entered a whole new realm of herps. Im betting this water monitor was WC, which your king and corn, im betting, werent. This means he could be carrying a whole host of parasites, so I would recommend a full vet check up. I would treat him with panacur. It is a very common parasite treatment and covers a pretty broad range of endoparasites.