This sounds exactly what I went through with my dog. Went through all the steps you described - and it came down to the treats. My vet said that there is often a lot of fat in processed treats (even rawhide can be included in this for some dogs), and that it can set a up a 'pancreatitis'. Basically pancrease can't keep up - so instead of neutrallizing the fat - it tells the tummy not to send any more downstream. Most often in older dogs - even if they've been on the same treat for years.

Once irritated the digestive system needs intervention, ie. a complete rest and tums usually does it. I'll bet if you stick to non-processed treats and no rawhide, you'll have no more incidents.

Good luck - - Millie's a lucky rescue dog to have found you!