Sewer/black tank smell inside trailer

I do what "N-Touble" does every now and then as well.... Just to make sure I get the black tank clean. After I put my treatment in the black tank, then I also re-fill the treatment bottle with water to get the residual treatment out, and pour half of the bottle into the bathroom/kitchen sinks.... "P" traps can start smelling if that water sets there to long. P traps are designed to keep the smell out for every day use.... not if they set up for awhile.