We are always pretty frugal with our grey water and use Tubtrugs for dish washing so most of our nice soapy water is then dumped down the toilet to the blank tank. We decided to practice extreme water management recently while boondocking just as practice for our two weeks in Acadia with no hook ups. All went well until we kicked the propane water heater on for a few hours (we only turn the hot water on when needed).
We started getting a really funky smell that we tracked down to under the sink where the hot water heater lives. We were afraid something melted, burned, died, decayed... something! After dismantling the lower cabinet around the hot water heater (and finding a nice wrench that Phoenix lost in there) and much sniffing of random objects (definite ewwww), we guessed it was the vent for the drain pipe. We figure whatever sludge was in the pipes got heated up and fermented or did something and the odor was wafting out the little metal screen under the sink.
We promptly dumped a bunch of baking soda down the drain and hit it with vinegar to bubble out as much as we could. It still smelled, though not as strong. We then filled the sink half way with warm water and put in about 10 times more Dawn dish soap than I usually use and pulled the plug to give a good whoosh down the drain. When we got up this morning the smell was gone! We will try to remember to do a good drain/trap cleaning before we boondock to avoid the stink in the future.
So if anyone gets a weird smell under the sink (but not from the top drain), try the super cleaning tricks before you dismantle your cabinets. It's so much easier.