Thank you all for your input.
After weighing my choices I've decided not to have the surgery done. I talked to the vet about my decision and she said "you are not making a mistake." I feel a lot better now.
The point that made the decision easier for me was the surgery would not make a difference to Max's limited life span. (You guys helped me with that part)
The vet stressed that I shouldn't feel rushed. She said I should take it day by day and decide accordingly. She told me he wasn't in pain but to watch out for depression. She made arrangements with me, at my request, to prepay the euthanasia fee. I wanted to be able to go straight in and out.
Try not to judge me too harshly but I can't participate in this.
My dog knows these people very well. He's diabetes causes him to be at the vet's often. The receptionist also his groomer/handler will stay help him throughout the ordeal. She's the one who helps him through the glucose curves for his diabetes. I think this will feel the most normal for him.
The vet assured me that she uses a very high dose of sedative prior to the ordeal. "It is quick" she said, "he won't feel any pain."
I'll take her advice and take it one day at a time. I suspect it won't be long though. Histiocytomas develop rapidly and are very messy. I understand why the vet warned me the dog might become depressed because of them.