Steve - the holding tank is about 14 gallons - you can measure the dimensions and calculate the volume. Think of your 25 gallon analogy above. Cut a 55 gallon drum in half (take the smaller half) and that would be about the size of a tank you describe. No way that sucker fits in the v-berth!
We're talking about a 10 pound differential here on dry weight between a bladder tank and a poly tank. If you store a gallon of fresh water (or beer) that's pretty close to the weight differential. You'll get more mileage out of keeping the tank pumped dry.