I would start with the basics and eliminate all potential variables, one by one.
Give them a deep, usable substrate of a sand/ soil mix. I have found that most commercial topsoil does not suit them well. I am using river sand mixed with a small amount of clay.
I imagine the lizards are dehydrated. I have seen a number of pet shops keep their lizards in similar ways and have varanids with poor body mass and tail mass. They were all desperately dehydrated.
Are they kept in enclosures with a screen top? Ensure they have a small water bowl, I use tupperware, and a deep substrate, 12 inches or more, so they have moist/ humid retreats. It is hard to give them the proper substrate in an aquarium.
The basking temperatures are too low. 100 degrees is not sufficient. My pair have access to basking sites of 120-150 F. They use this entire range.
Adjust their husbandry so it is satisfactory and go from there. Most monitors die from complications of poor husbandry.
I would fix what you presently need to fix and continue evaluating them.