My sympathies Claire....it must be very frustrating and worrying for you.
Looking at the pictures - firstly the skin looks very very dry and the fact it is sloughing off in large areas would prompt me to consider whether there is either a bacterial cause (specifically staphylococcus or pseudomonas) rather than fungal......or in view of the number of different treatments already applied, whether the skin has developed an allergic reaction to one of them....I would be particularly worried about the use of pure/neat tea tree here as that is very harsh and can cause skn irritation/burning/blistering.....siggest in any case yo stop using this) Or possibly perhaps a combination of shampoo and topical has caused a toxic concentration of medication to build up.
Many of the treatments used could affect/traumatise the skin and reduce it;s ability to fight off other secondary infections, especially if used frequently.... and it could be that this is what has happened...Also, since a wide variety of treatments have been used over only a 5 week period, there could also be build up of resistance by the micro-organism concerned because no one treatment has been used for long enough, ....or toxic interactions (or toxic accumulation) of meds...or an allergic response developing.
So having reached this stage where treatments aren;t working, personally I would now stop all baths etc for a few days (check with vet how long) in order to get skin cultures done to identify the real cause and their antibioitic/antifungal sensitivity to determine the best treatment. I would want to know if it is
a) fungal (and is it ringworm or is it some type of yeast - or even histoplasmosis)...in which case I would consider an oral antifungal
b) bacterial in which case maleseb treats bacterial and fungal skin conditions. I would be wary of oral antibiotics since in this case I do not know which would be best as most of the ones for piggies are designed to keep the Gram positive flora intact and destroy Gram negatives whereas a lot of bacterial skin conditions can be Gram positive. There is an antibioitic cream - Polymax which I have been prescribed for my piggies before that keeps chronic wound infection at bay
c) an allergy - only (and only) if it is an allergy would I consider a steroid injection to reduce itching as it supresses the immune response..(.if it is an infection then a steroid injection could do more harm than good) Stopping all treatment for a few days and seeing if it calms down would also help identify if there is an allergic reaction going on.
However this approach of stopping all treatment to enable cultures to be taken is risky in view of how bad Rodney is. If you don;t feel able to adopt the above route to identify the cause - (suggest you discuss it with the vet) ....I would suggest you at least opt for oral griseofulvin or itrafungol.....and either malaseb shampoo for bathing (as it is anti-bacterial as well as antifungal) or perhaps instead use a polymax antibacterial cream as there are some pretty bad open wounds in the piccies....
Whatever treatment you decide - you need to stick to it for probably 4 weeks at least... and don;t use/try anything else.....and keep en eye open for signs of dehydration as this can occur with persistent large area skin infections
Good luck...really hope you can find the right treatment for Rodney.
x