Hi Mary,
This article would be a good place to start for learning on how to do a proper food trial, they are very difficult to do correctly: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2499 Your vet should be able to help you. You need to clear the infections in order to know if the trial is working. Antibiotics and anti-fungal treatments are often necessary as secondary infections are common.
Doing a proper trial is $$ in a large breed. Start with a good diet history. Ingredient sources in the new diet should not only be something your dog hasn’t eaten before but ideally not closely related to ingredients that have been fed before. For example if the dog has been exposed to beef you would likely want to avoid bison. If the dog has been exposed to chicken you’d want to avoid turkey or to be safe any type of fowl.
When doing a trial only use foods from the vet or homemade (you must be very careful not to cross contaminate). This is where the cost comes in. Limited ingredient diets found in stores have been found to have protein sources not listed on the bag.
Good Luck
