Skin issues with cats

I have 5 cats. All of them are indoor only. I have been struggling with figuring out why two of my cats are having skin issues. I have taken them to their regular vet along with a specialist dermatologist vet. They both have been treated for fleas even though I have not seen a single flea on any of my cats. (I flea comb them all at least once a week). They are on special food Royal Canin Rabbit and Pea prescribed by the vet. The vets have tried antibiotics, anti fungals, steroids and nothing has seemed to help. I have no changed their litter in years. They use Fresh Steps unscented cat litter. I have tried fish oil, and now am trying Dinovites which is a blend of vitamins and probiotics. I’m getting desperate and running out of ideas and things to try.

One cat has been licking all his hair off and seems to be itchy. The other cat has a bunch of scabs on her face and the base of her tail/lower back.

Has anyone dealt with this/found a solution?

The vets are just bleeding me dry at this point.