I'm so sorry to hear about Coach.
I don't know much about stomach cancer in dogs. My bullmastiff was diagnosed with lymphoma at 6 years old and with treatment (chemo) he lived a month shy of his 8th birthday. I will say if you decide this is something you want to treat that a specialist is probably the way to go. We saw an oncologist for my bullmastiff, and I truly believe he received better care than his regular vet could have provided.
You will both be in my thoughts.