Bought a gallon of DigestIt in Aug 2013 to use in the motor home and in April 2014 we took a trip leaving the motor home behind. I emptied the motor home tanks before we left and put a good amount of water with a BIG double dose of Digest it into the Black Tank. Came back to the motor home in September 2014 and the Black tank was smelling badly. It had been an unusually hot summer in the NW. Maybe I didn't use enough of it per treatment? 1 gallon was all I bought and used from August 2013 to April 2014.... about 32 weeks. I poured a good amount down into the Black tank, probably 12 oz, after I dumped, flushed and rinsed that tank when we got back. ( I use a white plastic solo disposable cup to measure... I figure that's about 4 oz. Directions are 4 oz per 40 gallons ). After two weeks, It did NOT tame the smell.
Switched to DRAINBO in October and it really did do a better job at knocking down that odor to NOTHING in one treatment. 1/2 gallon is supposed to do 10 treatments ( bought it at the local Ace Hardware) and I've been happier with it... but it's winter time. So it's not a fair comparison since warmer temps bring on stronger odors. I've been putting 1 to 2 Dixie cups into the black tank after dumping/rinsing/flushing ... and I've still got about 6 dixie cups worth left of the 1/2 gallon. I'm about ready to go buy more.
Maybe the solo cup I'm using is a 3 oz cup. There's 128 oz in a gallon.
If I had really used 4 oz per week, that gallon of DigestIt should have been gone in 32 weeks. I still had some left. BUT I'm using that same cup with the Drainbo and putting the same amount down into the tanks and it's working a heck of a lot better than the DigestIt. (JMHO)
We have not moved the Motor Home since October 2011. Standing still creates it's own challenges to keep things odor free. Just water don't cut it. From what I've experienced, I'd recommend Drainbo over DigestIt.