I am sorry that you are still having this problem. Coincidentally, my friend has had very similar problems with her Labrador. Like yours, the dog has had campylobacter and another parasite which she cannot remember the name of. Treatment has been given and the dog's recent stool sample was clear but has since had intermittent problems with poo. She has recently gradually changed to cold pressed and all was going quite well but yesterday the dog became ill again with diarrhoea and vomiting. They are going to the vets this afternoon.
We are tempted to think it is dietary and I can see why the vet thinks so but maybe it could be more to do with the residual effects of the disease. My friend thinks that her dog hasn't been treated for long enough with erythromycin - think she read on the Internet that some vets give it for three weeks. Also, as the gut has been inflammed, maybe it makes the dog more susceptible to infection.
I agree about the fish and potato being potential problems but the only way to be sure about sensitivities is to do a proper exclusion diet and that can take some time and determination. I am not sure of the exact protocol - be guided by your vet. I understand that it begins with one of the hydrolyzed protein foods and then you add in different proteins to assess the dog's response.
If you don't want to go down the route of a controlled exclusion diet, the only thing I can think of is to use a quality wet food, minus fish and potato just for the moment. This means that you will have to avoid grain free. Wet foods tend to have simple recipes so it is easier to identify problems.
Naturediet might be somewhere to start - check out the recipe for their chicken version here.
They have some more information here
but you could telephone for further advice if interested. They used to do some good fact sheets on intolerance but don't know if they are still available.
Naturo wet also has simple recipes but you need to stick with single protein at the moment so probably avoid the ones with two types of protein.