Huh?
Why do you have to dissassemble the v-berth? There should be a deck fill.
If the setee tank does not empty, the valves are probably set to the v-berth tank only. Locate the valves and open the settee tank line. -or- if both valves are open, sometimes one tank will empty first, depending on line pressure, and the other tank won't empty due to air in the line from the 1st tank. In this case, you have to close the the already empty tank line, and keep the full tank line open.