There are studies showing that CoQ10 can lower creatine and bun levels. It also lowers liver enzyme levels. The amino acid L-threonine will help her retain muscle and is supportive for the heart and liver. Taurine of course is important for healthy heart and eyes and can become depleted in the ckd cat.
I would get some kidney glandulars (swanson vitamins, ancestral supplements are both good sources) and give her some of that daily along with 100mg-200mg of coQ10 (get the powdered form in capsule so you can easily put it in her food or dump in some water and pull into an oral syringe for dosing) to support her kidneys and take her back within a couple of weeks to see if that is helping.
I would give her 1/4 tsp of taurine (amazon bulk supplements taurine is what I use) and 1/4 tsp of threonine daily in her food too.
You could easily start making her food too. Lots of info online for that. But you'll need a good vitamin/mineral supplement to make sure she has all the nutrients she needs. I believe the optimal ratio for cats is 80% protein, 10% carbs and 10% veggies, at least it was several years ago. I don't know if it's changed. But you could easily boil or bake a whole fryer chicken and cook some chicken livers; and hearts (you might have to order them online from a small organic farm), a bit of brown rice and some veggies. Throw in a food processor to puree it all after it's cooked. You can freeze meal size portions in ice cube trays and then store in a zip lock bag in the freezer for easy portion control. Just take out the portion to thaw in the fridge overnight for the next meal.
I would stop all vaccinations at this point. Because at 13 years old her body shouldn't be subjected to them anymore. If she's an indoor cat she doesn't need them anyway. Did you vaccinate yearly, btw? Over vaccination has been linked with renal problems.
And if you have to leave town, I would get a pet sitter to come to the house twice a day to feed and visit with her to help keep her stress levels down.
Rx Vitamins has an amino b-complex that is very palatable. I give this to my Patches who is 14 years old. I found this liquid vitamin A for her too. She gets one drop daily.
They have a vitamin D liquid formula too. If you're going to go the homemade route you'll need to supplement if you don't give her any liver. Liver stores vitamin A and D.
You can order plain aluminum hydroxide powder on amazon to bind the phosphorus too. But if the coQ10 brings her bun and creatin to normal levels, you may not even need to do that. I know 100mg-200mg coQ10 sounds high but I have given my Patches 200mg coQ10 to bring down her liver values when she had hepatic lipidosis and she tolerated that much coQ10 just fine and it quickly brought down her bilirubin levels too. I was also giving her milk thistle, lecithin, SAM-e and medicinal mushrooms.