I also think you should drain and re-fill before you leave. Â Your ozonator and silver (do you have that) should keep the spa clean and clear for you when you get back. Â However, I don't think you need to overdo it with chemicals. If you have a good CD ozone generator and silver cartridge, you'll be fine.
I would not drain and leave empty. Â Spas want to have water in them. Â