DITTO on all the suggestions posted thus far. We've included most of them over time in our rigs.
One thing we recently ran into on our SeaLand flush toilet in our 5th wheel (we have a dry toilet in the van conversion so it is a non issue there) is we did not notice the build up of solid residue on the sides of the down pipe from toilet to black tank below. We had all the right things taking place such as good venting and water-trap in bowl, but we began to notice an overwhelming sewer odor during bowl flush once our weather turned a bit warmer on the oregon coast (this week was the first week of 70 temps since last summer). We were puzzled and then i thought of taking a flashlight and looking down the open toilet and saw all the solid build up on the plumbing. Soooo, my MacGirlver of a wife grabbed a bunch of paper towels and rubber banded them onto our toilet bowl scrubber wand and i held open the toilet-flush while she scrubbed the walls...
WaLa!!! No more smell
The simple things in life are so grand at times.