This sounds like a variation on the idea of a cesspool. Your system apparently uses a redwood box instead of a pit. I assume that the box is pumped of solids on a regular basis. The basis of operation of this system should be that the water drains out leaving the solids. The system should stay essentially dry which will allow the system to stay odor free. If the box fills with water, the system will turn anaerobic and odors will increase. 
If the box has standing water, then it should be treated by pumping the solids out. Septic Scrub can then be added along with enough water to dissolve it and allowed to filter into the soil. I would suggest adding two bottles of Septic Scrub and then the next day adding two more. This will remove the build-up of anaerobic by-products and help increase drainage of the soil. 

If the system is esentially dry, I would not treat it with anything. I would suggest however that you keep the system pumped.

If you expect your water usage to increase from the previous owners, I would monitor the system carefully for an increase in the water levels and retention times in this box. It is not uncommon for systems to fail with new owners, especially if water usage and composition of waste changes.

If you have any questions, please do not hesitate to call.