Personally, I would suspect the steroids are at the root of it. Just about everything you’ve listed can be a side effect of them.
Steroids can increase water consumption and lead to weeing in the house. They can also cause stomach issues for some dogs, in which case a stomach protectant like omeprazole should be prescribed alongside them. Lethargy is also a common side effect. These things are often dose dependent, but some dogs are super sensitive.
Are the places she’s choosing to lie cool? Steroids can also cause dogs to feel excessively hot. Even my skinny, thin skinned sighthound used to choose to lie on the floor rather than his bed when he was on them.
How long has she been on them? Long term steroids can lead to muscle wastage as well, which might explain the problems with her back legs. As for the destruction, anxiety driven behaviours are also not unusual for dogs on long term steroid treatment.
To be honest, I would book an appointment to discuss it with her vet. With steroids, if they are essential and long-term, they need to be carefully and very slowly tapered down to the lowest possible dose and frequency that keeps the condition under control. Sadly for my boy, long term steroid treatment caused him to develop other serious health issues, even with very careful management. He had a really serious life threatening condition though, so there was no other option. In your position, I would want to discuss all potential treatment options aside from steroids with the vet to be sure there is no other choice. If she does come off them, the taper needs to be ridiculously slow and carefully managed by the vet.