I think you need to find out what the vet means when she says to "wait it out." What are you supposed to be waiting for? Improvement? If so, what kind? I've never heard of a tumor healing on its own, so if she's suggesting that it's going to get better, then it seems like the tumor isn't cancerous? Or is she waiting for your kitty to suddenly worsen so that you'll have to make that awful decision?
Regardless, since the vet mentioned quality of life, and the lack of appetite could be due to pain, as lcordaro mentioned above, there's really no reason for her not to prescribe a pain med. And if she's thinking that your kitty is nearing the end, that's just all the more reason for her to do everything possible to keep kitty comfortable. Ask the vet for buprenorphine; it's more effective than gabapentin. If she will only give you meloxicam, just be aware of the potential risks and insist on a really, really low dose.