Varanids - almost more than any other reptile - need to have their husbandry parameters religiously met.
They need HOT basking spots in order to support proper metabolic and renal functions. Most people do not keep them anywhere near hot enough.
They need humidity to prevent dehydration. I'm not talking about soaks or pedialyte or misting. I'm talking about burrows that - through allowing them to breathe moist air - ensure and maintain proper hydration.
They need to be allowed to feel secure.
Once those requirements are met, they will eat and thrive.
When rehabbing an improperly kept varanid, these three things need to be provided. No food, no handling - just a HOT basking spot, a deep substrate capable of supporting the proper humidity and security.