Joy, as one who has been doing the allergy thing for 12 years now, I know where you are coming from.  I want to share one thing our vet asked me to do when we decided to do chemo on Trouble.  The first thing I wanted to do was get her on a healthier eating plan, but he specifically asked that I not change her diet until after the chemo was finished.  His concern was that the change in diet could cause a reaction and we wouldn't know if it was the chemo or diet.  You may wish to have this discussion with your vet.

If you still want to try a natural diet, here is a website with several different choices - Cedar Creek Veterinary.  One of the diets is the reducing diet I have had Trouble on since July.  After she hit her target weight, I just kept increasing the amount until she stabled out.  I add a supplement, fish oil, and glucosamine to hers.