Check your temps. What are they?

Your sav should absolutely be digging. They all dig, and you don't own the exception to the rule. It's possible the enclosure is so consistently cool that it will halt this behavior out of necessity.

The other reason I suspect the temps are off is your description of its body weight. Bloated body, thin tail... That is a telltale indicator of an under-heated, overweight monitor.

I have nothing at all against veterinarians, but I doubt bringing your lizard out to see one will do any good. Instead, you should go check the temps.