Despite what I said to CF above, I do have a soft spot. I had a cat a few years back that I treated just like you have treated yours. Put out the food and water, have a friend check once or twice while I'm gone, and go off on "walkabouts." I come back, the cat's still there. It's all good.
He died a few years ago of CRF. He went from 13 lbs to less than 6 lbs. over a year and a half. I kept him in good spirits with sub-Q injections of lactated ringers solution (3x/day) and special, low-protein cat food (kD diet that you get from the vet). He carried on after his diagnosis for the better part of 18 months or so, when the vets all gave him less than 6 months to live. I had people telling me to put him down, but he still seemed to be a happy kitty, so I let him cost me thou$$and$$ of dollars over that time period. He became the "Poster Child" for CRF survivability at WSU, where he made more than one appearance at the vet school there.
Why am I telling you all this? Have you had Kitty to the vet recently and had any blood work done on her? If not, now's the time, if you really love this critter. If she is losing weight this fast, she may have CRF. It is imperative to get her on a new diet (with reduced protien intake) and to get some fluids into her. You may even have to spart feeding her with a syringe. It is not going to be easy, and your climbing is definitely going to suffer, unless you can find a good petsitter (like I did) to administer the meds while you're off climbing.
And what do you mean exactly by "siezures?" Are they "classic" siezures, or do they look more like the result of muscle atrophy and the resultant failure to control them? Let me know what the vet says. If it's CRF, I can help you with tips and links to good info. Seriously. I really am a softie for cats, although it would appear to be against my nature.