Black tank stench won't go away

If there are no leaks, I would suggest not emptying the tank quite as often. Use lots of water to flush each time and wait until the tank is half or three quarters full before dumping. That allows enough water to accumulate in the tank to break things up and flush more thoroughly. Check the seal at the flush valve. Is the bowl holding water between flushes? Rubbing a toilet brush around that seal and putting a little vasaline on it helps. Assume, since you've been living in it for all this time and it only started recently that you are using sufficient chemicals. Also, might check vent pipe to be sure it's not plugged? Is the bottom of the trailer enclosed? Might be a leak ih the sewer line?