A month ago, I left my spa unattended for 2 weeks for the first time, and there was a non-zero chlorine reading when I came back (somewhere between 1-2ppm). Water was crystal clear with minimal chlorine demand upon my return. Here is what I did:
1) In the days prior to leaving, don't use the spa, leave it at normal operating temperatures (chlorine is more effective at higher temperatures), remove headrests, do a thorough cleaning of your filters including overnight soaking in cleaner solution (reinstall filters after), and unplug your ozonator if you can (ozonators consume chlorine, and good high-output ozonators like the Bullfrog EOS consume an enormous amount of chlorine).
2) Set pH to 7.2 (the lower the pH, the more effective the chlorine, but don't go below 7.2).
3) Once to twice a day, dose with liquid/pool chlorine to 10ppm FC with jets on low (high jets may cause chlorine loss to the air and pH increase via aeration). Each time you dose, you should need less and less chlorine to hit 10ppm - your chlorine demand aka CD should be lower and lower each day. Wipe above the waterline, the top ledges, and the underside of your spa cover with the superchlorinated water to kill off any bacteria hanging around. Check your pH, add acid as needed to keep it at 7.2. When your chlorine demand stabilizes at a low level, your spa is well sanitized/disinfected.
4) I can't easily unplug my ozonator, but I know that the Bullfrog ozonator only engages during heating and filtration cycles. Therefore, I set my heat to as low as it goes (since you're doing this in the winter, maybe set it a little higher if there is risk of power outage/spa failure), and I deleted all of the filtering cycles. The Bullfrog ozonator likely still runs during the automatic periodic temperature checks, no way to stop that without unplugging the ozonator. Figure out how your ozonator operates and disable it as much as you can.
5) Let the spa sit still for 1-2 days with a high dose of chlorine in the water. Now that there is no organic material anywhere in or near the water, and no ozone to eat up the chlorine, confirm your chlorine demand (CD) is still low. If not, you need to biofilm purge or otherwise clean the spa better, or get a friend to come over every second/third day to pour in some chlorine.
6) Minutes before you step out the front door to go on your 2 week trip, dose one last time with more chlorine, depending on your CD. If your daily CD is on the high side, then aim for a higher number like 15-20ppm FC. If your chlorine demand is low, shoot for at least 10-12ppm.
Assumptions:
* Spa was already thoroughly purged with Ahhsome (including drain line - open the drain, stick a hose nozzle into the drain port in the footwell and blast away) to eliminate chlorine-consuming biofilms on internal plumbing. Without a thorough biofilm purge, your CD will be higher than it should be.
* Cyanuric acid between 0 and 20ppm. Spas that don't see sunlight don't need any CYA. The higher the CYA, the less effective chlorine is at its job, which is to sanitize/disinfect. If above 20, you'll need to increase the 10ppm mentioned above to a higher level - use PoolMath calculator and look at the SHOCK number based on your CYA levels and other water measurements. If 40 or above, I'd drain half and refill to get closer to the 0-20 range to ensure high chlorine effectiveness.