It of course depends on how many are using your facility. For my wife and I, just the two of us, six days is about it, but the tank on the Gemini is probably a bit smaller than yours. Our first couple of times out, it filled in 2 or 3 days, that was because the bathroom sink drains into the black tank, didn't in our previous RV. We are now very careful about using the bathroom sink. Since whenever we are out, 97% of the time or more, we are connected to sewer, its not much of a problems for us. Not sure I would want to mess with a portable black tank, in fact I'm pretty sure I would not.
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