Someone in these forums gave out a pointer for flexible tanks as follows:
One thing I noticed about the flexible tanks is that they seem to have 1-1/2" as the largest input/output connections on them. It is my understanding that they are typically used with a toilet that has a macerator pump built in. Also note that as far as I can tell the marine systems have a combined gray and black water system.
On my 1973 31' Sovereign
mid-bath the black water tank is above floor. Someone added a rather poorly installed grey water tank under the floor in the cavity just behind where the black tank is. I have been considering various options for black and greay water as part of my total remodel. I want to eliminate the black tank above the floor where it is since it sticks into the back bedroom and gets in the way of my floor plan ideas for back there. I have all but made up my mind to go with a 40 gallon combined black and grey tank under the floor just below the toilet location. I would install a macerator pump on the output. There is a lot of mixed reaction to the idea of combining the black and grey. For some more details of the discussions my posting on the topic prompted look at the thread titled:
Combined Gray and Black Water?
I would be happy to discuss the approach I am thinking of with anyone that might be interested. I also have various details about where to get a tank that will fit the area and how I intend to support it under the floor.