Lauri - we are in virtually the same position as you, with Wallace. He is probably around the same sort of weight now as Miles and he too looks dreadful. He has the sunken eyes that you describe too. Wallace was diagnosed with a brain tumour a couple of months ago and he also has a problem with a couple of discs in his neck. He has muscle tremors which we did get under control with Tramadol when he first went onto it byt now, even though he is having 6 tramadol tabs a day (300 mg in total), the tremors are back. Nobody knows if the tremors are tumour related or neck pain related. He's seen vets, specialists, had an MRI scan (tumour is inoperable and even a biopsy isn't possible due to its location).
He does eat now (thanks to Jillysfullhouse and her gift of Nutritional Yeast) but he's not gaining weight. He's still losing it in reality.
I was talking to the vet the other day and we've decided to add in 75mg of Rimadyl daily, plus we have Gabapentin on order so he'll be starting that this week too.
Other than that, there's nowhere else to go.
Wallace still wants to go for a walk, even though he can't walk as far as he used to and he still loves to get treats and yesterday he picked up a stuffie for the first time in ages, so I don't feel that we are 'there' yet.
This post hasn't given you the answers you're looking for but I just wanted to let you know that we understand