Sounds like a rough patch, glad she has done so well for so long with renal failure. If it is a sudden change there be something else going on too, where it is worth it to check her urine and blood values early just to see. Is she on any medication or fluids for renal failure? Is she going full volumes outside the box, because she has a hard time moving and getting to the box or is it small amounts more frequently? I would think about her quality of life overall- eating, drinking, does she seem happy, getting around normally, does she still want the same amount of attention or is she hiding more? Some will say as long as she is eating and drinking she is fine, but as you know that isn't always true.
It sounds like you have a good relationship with your vet, so maybe talk to them again discuss quality of life and signs to watch for if she is in any pain. Also if she is arthritic at all getting into a litter box can be very hard on her, which may be part of the reason she is going outside the box and pan. If you haven't had blood work checked recently I would have that rechecked, maybe a full panel, and her urine checked with an exam to make sure nothing else is going on too. I have know cats that used buprenex for pain and it made a big difference in very low doses. Metacam is probably not an option for her since it can really effect kidney funtion. Cosequin in a glucosamine supplement that can help if there is arthritic pain too. Fluids under the skin that you can do at home can help sometimes as well as other medications if she isn't already on something.
So I guess talk to your vet again and see what options are available. In the end only you can make the decision and I know it is a very hard one to make. Good luck to you and your kitty and I know you love her and that is why you are really thinking about what to do.