Correct. There is also the possibility of using a pop-off valve as used on steam boilers set for something like 70 pounds of pressure -- located near the cabin, of course. This would assume the occasional spurt of waste water can be accommodated.
OT --I was once assigned as maintenance engineer for a group of irrigation projects. At one location we had a buried 39 inch pipe, a mile long, from a reservoir to the head of a canal. Failure of the local crews to observe operating procedures at the springtime turn on of the system resulted in water hammer that blew off three manhole structures along the length of the pipe. Fortunately, no pipe was broken and we were able to get the system back in operation before water was needed. But we should have had some kind of custom designed pressure relief device built in near the end of the pipe.
SJF