Pannel to Holding tank
If your 340 is set up the same as my 336 (and I think it is) the large center panel aft of the bunk in the aft cabin comes out with the removal of four screws. That will give you clear access to the aft tanks, fuel and holding. Just move the mattress out, take out the four screws and that panel across the rear swings up (as I recall) and out. That gives you a clear view of the tanks. If you need to get to the sending unit (which I recently replaced), you will need to come in from above through the starboard aft lazerette. There should be an access opening under your batteries in the bottom of that locker.
