Hi Corinne,
Sorry for the delay! It's been a hectic start to the year! Regarding your case, the improvement on cooked real food (and thus the implied worsening on raw food) suggests Spleen Qi or Yang deficiency, perhaps causing Dampness which may in turn be instructing the descent of Qi. That puts me in mind of Wei Ling Tang, or even San Ren Tang (given the response to Pepcid, which has a cooling effect on the Stomach).
San Ren Tang is also a better choice given the coughing and the purplish tongue. Of all the anti-Damp formulas, it's the one that best fits a toned pulse and a lavender tongue. That is because it addresses Qi stagnation arising from the obstructing Dampness. Qi stagnation can also cause that bowing behaviour you observed which occurs when a dog is trying to stretch its abdomen (and the GI tract therein).
All told, then, it's how I'd start. If we need more warmth, we can add Wei Ling Tang, so keep that in mind. If there is more inflammation and colitis, we can add Si Miao San, though SRT can take care of that, too.
Hopefully this helps you out!