It sounds like you are doing everything possible for your daughter. I would suggest that your doctor should test these lesions, or culture them, to see what is causing the rash (if he hasn't already). Because there are blisters-- it sound like a bacterial infection of some sort. Impetigo, a fairly common skin infection especially in children, causes these kind of blisters with redness and crusting. This is not normal diaper rash, or she would have healed weeks ago, especially with the type of care you are giving her.
The other thing I would suggest, especially if she is not potty trained at all (you say she is a toddler), is to change to disposable diapers. Some children cannot tolerate the moisture that is held in a cloth diaper. Disposable diapers have a gel substance in them than wicks the moisture away from the skin, and with frequent diaper changes can actually keep the skin very dry.
Here is a link to more information about Impetigo. If you click on the link on the left side of the page for skin rashes in children, there is a picture of an impetigo lesion on one of the slides.
So to sum up, I would suggest that you see your daughter's doctor again, and ask him to test the lesions for possible bacterial infection. Actually do a culture and sensitivity. This way, if it is a bacteria the test will also tell you what antibiotics will work for it.