Odor Problem with Sealand Traveler - SOLVED!
Short answer: if your toilet has overflow holes in the bowl, regularly (monthly?) fill the bowl enough to allow water to flow through the overflow holes. The overflow holes have P-traps which can dry out and allow odors from the tank to invade the interior of the coach.
If your coach is likely to have freezing temperatures in the toilet, but sure to use antifreeze or get all of the water out of toilet, including water in the overflow traps.
Long explanation: Since we have owned our coach, we've had odor problems in the water closet (private toilet room). Depending on ambient temperatures, "the stinkies" from the black water tank became offensive after three to five days. Turning on the exhaust fan made the problem worse. When I dumped tanks and used grey water to back flush the black water tank, the odor got worse. I always thought fumes from the black tank had a path back into the water closet.
Last night while searching the Internet for solutions I found an explanation of the structure of the the Sealand Traveler toilet bowl with overflow holes, and the solution to the problem. The overflow holes have P-traps. If the P-traps don't contain water, there is an open path for air to flow between the black tank and the water closet. P-traps will dry out. You must occasionally fill the bowl full enough to flood the P-traps in order to provide a block to air flow through the P-traps.
I filled the bowl and "the stinkies" from the black water tank no longer filled the room. I ran the exhaust fan, and the air got fresher rather than stinkier. I dumped tanks and back flushed the black tank today without introducing bad odors into the water closet.