I bought Chlorhexidine at the local feed store, $4.79 for a good size bottle of concentrate, you have to dilute it a LOT.
When I shampoo dogs, I take an old, washed out dishsoap type bottle (you can buy squeeze type bottles if you prefer) and put, I guess about a 1/4 cup of shampoo into a 16 oz bottle and fill it the rest of the way with warm water and shake it enough to mix. Then I squirt the warm, sudsy water onto the
dry dog and lather up.
This takes far, far less shampoo and is much easier then soaking the dog first. And this way you can lather from the skin out, not have to work the shampoo down from through the coat to the skin.
As always, rinse well. With a dog with bad skin, rinse twice as well as you think you need to.
When I lived in FL with my (now gone) Thunder, that poor dog was allergic to EVERYTHING. Including fleas (nasty and year round in FL) and every flea treatment, shampoo, spot on, spray, pill, house treatment known to man. Weekly baths were the only answer. I used this;
http://www.petsmart.com/product/index.jsp?productId=2754478 I'd lather him up, leave it on him for 10 minutes and then rinse and rinse and rinse. It was very soothing and healing to skin that was aggravated by everything and the pennyroyal repelled the fleas for about a week. It sounds extreme but if the dog is bad enough get a rainbarrel and rinse him with rainwater. That avoids any chemicals in the water and also salts from a water softener.