[bryguymi]

Bryan,

My toilet is in middle of coach and the waste tank is directly under the toilet.  The pipe from the toilet goes essentially vertically into the top of the tank.  In my case the seal between that pipe and the tank was not airtight.  The square tanks that were used on our vintage coaches seem to sag in the middle of the top and this pulls the tank away from the pipe.  It seemed that I always had a bit of a smell.  A little clear silicone around the pipe did the trick for me.

bill

Bill, 

My toilet is also in the middle of the coach, sounds like the same exact orientation/setup.  I'll check that seal yet today, thanks for the tip!

Bryan