I'm definitely glad you're getting a second opinion. That was going to be my next suggestion to you. When my Brody was diagnosed with kidney failure, one of the symptoms was that he'd vomit about once a day. He was on Hill's k/d prescription diet for years before that as well. Once his levels went up as I said before when he started vomiting, our vet suggested we do Sub-Q injections. It relieved a lot of his symptoms. Run this past your vet but I'm not sure if it'd be recommended because his stomach is swollen and you already mentioned he had excess fluid. Brody didn't have a swollen stomach though and he stopped eating later in his diagnosis so his was different than what you described...
Did they tell you what his BUN level was?
Anyways, I'm sending lots and lots of prayers to Tucker. I hope you find some answers soon with all this. I know how hard it is
Good luck!