I really don't care what the inside of my holding tank smells like. I just don't want it to get out. And an activated carbon filter will consistently prevent that escape of odor without further ado. Other problems with salt water flushing aside, a handy person (even not so handy) with access to the vent line can construct and install an inline vent filter for <$20 with material from Lowes hardware and Walmart. PM me and I will send you instructions with pictures of what I installed.