I also have a cat in CRF, just diagnosed three months ago, my Ash the grey kitty in my siggy is around 13 but could be nearer 15, he was a stray, so the vets are only guessing,the time he was diagnosed was during his dental,his levels also were not that high either.

I also found getting him to eat was terrible, infact he would have slowly starved to death, my vet has prescribed Anaplex, which is a steroid,you may well ask your vets opinion about this, since then he is indeed eating much better, still has on and off days, but has gained a 100grms, so that is excellent, of course they are not without side effects, but in an older cat i think the benefits out weigh the side effects ,especially when you are dealing with CRF,my Ash also has a heart murmur, he is on Fortekor as well, which lowers the blood pressure and helps the kidneys, like i said he has ups and downs, and medicating him is no fun, but it is worth it,he is doing ok so far, and i really think it is what is keeping him alive and comfortable, he can still run up the trees and he is looking well considering, although he has lost a lot of weight, that is to be expected, the other benefit for Ash is he has arthritis and it helps with the pain of that too, anyhow something worth considering maybe.