Obviously the previous owner failed to dump properly and had some build up. So I thought I’d share what worked for me in this situation and try to help others. From the MSDS (material safety data sheet) Happy Campers consists of Zinc salts and unnamed minerals. Worked extremely well for me in regards to cleaning up the black tank.
https://www.amazon.com/Happy-Campers-Organic-Holding-Treatment/dp/B007S0LDME

