Originally Posted by Alex W
The amount that you need to fully pump depends on how long the lines are. On my 28' boat where there is a 3' of 1.5" line between the toilet and holding tank it doesn't take a gallon to flush, the volume of the line is just under a quart. Our 20 gallon tank is good for 6 or 7 days for the two of us.........
There should be a rise from the back of the head to a vented loop, to prevent back flow. That's what needs to be cleared, then it doesn't matter if the rest of the line is 100 ft long, if gravity does its thing. It's not the volume of the hose, its the amount of chase water necessary until it stops mixing and runs clear.
20 gallon for a week, if not using shore heads, is a seriously long time and far from typical.