I had similar issues with our bulldog. This time last year she had constant rashes, chronic diarrhoea and lots of hot spots, she got sick with parvo the week after we got her (fortunately she got better) which may have weakened her stomach and immune system etc. After trying lots of things we took her to an animal naturopath (www.greenpet.com.au) and she transferred her to a barf diet with some supplements. This was very successful. She is allergic to most meats other than lamb though and if she eats anything other than her special diet she gets diarrhoea immediately and not too long after a rash appears. Good luck I know it is distressing when they are uncomfortable and there is not much else you can do. Another thing we use is a shampoo and cream (raven pet products) you could search for it on the net. Unfortunately you have to buy it from the States but the combination of these have cured our pup. She is now 14months and i was dreading summer waiting for a flare up but she has been fine.
Best of luck