Sewer/black tank smell inside trailer

There's likely a turd teepee that has built up under the can from previous owner that doesn't get flushed by simply dumping the tank. As mentioned get a wand, or what I like to do is fill the tank about 1/2 water then drop a few bags of ice in and drive it down a long twisty road. The ice will break down the teepee and clean off any sensors. Take it to a dump station and watch all the pooh soup that comes out through your clear elbow.

Repeat this a couple of times then fill with a few gallons of water + treatment and see if your problem persists. I always leave some amount of water in my black tank as you don't really ever want it dry.