We're on a Bavaria 40 (2002 model) and it has two 10-Gallon (40 liter) holding tanks- one in each head. They are both gravity-draining. We are a family of 3 living aboard and are finding that the tanks aren't big enough (we get a pump out service 1x a week). We are thinking of replacing the aft holding tank with a bigger one since it is in the lazarette (we're in a twin-cabin model). Does anyone have any experience doing this? No idea what type of tank would work, etc. Also, we realize that we will need some holding tank monitors so we know what's in there.... I miss my compostable toilet on my old boat! LOL