Thanks for the additional information. You can check for pulses in her legs, but I think she may be ok there as it sounds like a neurological issue that is causing her trouble. On the inside of her rear legs, typically down the middle of the inside of the leg from the spot where the leg joins the body down to the foot is the femoral artery. You can place your fingers over the spot where this vessel should be (not your thumb since it has a pulse) and see if you can detect or feel the pulse, I am particularly interested if it is palbable or as strong in the leg that is weak and 'knuckling'. Knuckling is what it is called when the feet fold over as you have described, and it typically indicates that the brain is having trouble deciphering what the limbs are doing. In a kitty with normal conscious proprioception, the foot is immediately turned the right way when it becomes flipped in an exam. The fact that this is not automatically happening in your kitty can indicate an interruption in the messages between the brain and the leg carried by nerves. This could be a result of a brain tumor, inflammation, spinal injury, blood clot, normal age changes, or illness. It sounds like her rectal tone is also ok if she is able to consciously have bowel movements and urinate and is not incontinent, so that is a good thing. You will also want to feel the leg that is giving her trouble and make sure it is not much colder than the other one, as a lack of a pulse and cold feeling can indicate decreased or lack of blood flow and could mean that the limb will not be viable for very long.
I am ecstatic that she is eating well for you but I am concerned that she doesn't want to drink and that she tires easily (laying down to eat, having to stop in the hallway). If you are able to get her to eat canned food, that will be helpful in hydration because it contains a fair amount of water. You may need to start giving her fluids under the skin at home soon if she doesn't drink enough to hydrate herself. If your oncologist is able to do a bone scan or even an MRI for you I think you might want to discuss the merits of those tests with he or she. You also may want to inquire about a referral to a neurologist as well, because steroids can be very helpful in some cases and it may be cause for concern if you are not seeing results once steroid therapy has been initiated.
It sounds like you have been very dedicated and thorough in the workup that you have allowed for your kitty, and I am wondering if your vet ran a urinalysis at the time the bloodwork was done. I ask about this because a urinalysis is a much more sensitive tool for diagnosing kidney disease than blood work can be. As much as 75% of kidney function is lost before abnormal values are seen on a blood panel, where as little as 35% of kidney function is lost before kidney disease can be found on a urinalysis. While kidney disease is not necessarily the cause of the weakness and neuro symptoms you are seeing, it can be a complication and may be significant. While we are being thorough, a thyroid panel should be submitted if not already done since this can be a common ailment in older cats, and if untreated long enough can have some pretty weird symptoms that actually mimic neurological illnesses. Finally, an ultrasound may also be very helpful, because it provides much more detail than radiographs. It is possible for tumors to hide sometimes on x-ray, but a skilled and competent technologist will find a tumor almost every time. Ultrasound provides a 3-dimensional view of the internal structures, where radiographs provide a 2-dimensional view.
I am concerned that she cannot lift her tail as well, and want you to watch her very closely when she urinates and defecates. Cats are very funny creatures in that if the tail is painful and cannot be lifted they cannot posture properly to tend to their toilet habits, and will not go at all if it is too uncomfortable. If at any point she is unable to urinate or defecate, she will need to see an emergency vet right away so that she doesn't become septic or experience a ruptured bladder. I am also concerned about her pain level, and if she is not on any pain medicines, I think that this should be corrected ASAP. I do think that she must be experiencing some discomfort at this point, because if feels funny when your leg is not doing what it normally does, and the same thing goes for the tail. She must be feeling kind of weird and I think that any discomfort should be managed as soon as it can be.
You have most likely already spent alot of money on your kitty, and I know that I have just suggested a slew of very expensive tests (sorry!). Sometimes it is very difficult to find out what exactly is going on and that is frustrating. I appreciate your concern for your kitty and your willingness to go the extra mile for her. Please keep me posted on her progress and what you find out, and let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. I will keep you both in my thoughts, and wish you luck....