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Hello. I am sorry to hear your girl is having some trouble. I apologize that no one has assisted you sooner. I just signed in and saw your question.
When I hear of a shepherd or shepherd mix with a history of SIBO, gas and yellow loose stool the first thing that pops in my mind is a condition called EPI. This unfortunately is a condition that some vets are not very experienced with and one that is very common in the shepherd breed. There is a very specific test that must be done for this called a cTLI and I would suggest having this done and sent out to Texas A&M as they are the leading authority on this condition. This is caused by a lack of proper digestive enzymes being produced by the pancreas and pancreatic enzymes must be added to the diet. Here is a link that goes over this in more detail LINK
Parasites/protoza infection can also cause this so if a stool sample has not been sent to the lab for testing I would do that as well.
Having raised Shepherds for many years my gut tells me this is an EPI issue so I would discuss this with your vet as soon as possible.
I hope this helps give you some additional options to discuss with your vet. Please reply back if you have additional questions and I will be happy to continue. If you feel all of your questions have been answered please take a few moments to provide a rating so that I am compensated by justanswer for assisting you this evening.