[JRTJH]

First, what brand and model toilet do you have?

Thetford toilets are prone to leak at the bowl gasket.

Dometic 300 toilets are prone to leak at the bowl/base seal (factory seal not normally replaceable by users).

Dometic 310 and 320 toilets can sometimes leak at the bowl gasket. If the toilet won't hold water in the bowl, suspect this gasket is not properly seated.

Now, as for toilet smells. Many people turn on the vent fan when they start their "process" and leave it running after they are finished, so the smell can completley be drawn out of the trailer. Turning on the vent fan creates a vacuum in the bathroom and when you open the toilet ball valve to flush, you pull black tank odors through the roof vent and up into the bathroom through the open valve. So, always turn off the vent when you flush and then turn it back on.

Sometimes, if the wind is blowing the "wrong direction" you can open the roof vent and the wind blowing across the trailer roof will pull odors up through the vent stack and then push them down the open roof vent (if the fan is turned off). So either way, there's a possibility of black tank odor coming into the bathroom via the toilet/vent fan or via the vent stack/vent fan opening....

As for chemical treatment, there are several that people use. Ask anyone what works and they'll tell you the one they use is "the ONLY one that works"... Honestly, if the toilet is functioning properly, the vent stack is functioning properly, the vent fan is used correctly AND the wind is blowing in the advantageous direction, there's really no "stink from the tank".....

A couple other things you mentioned that you might want to take a look at:

You said you always dump the tank at the end of every trip and every time you're connected to full hookups... You may be empting the tank too frequently. If the tank isn't full, there won't be enough "rush of water" to draw the solids off the tank bottom. So never dump a half full tank. If you are leaving the campground and need to dump, then ALWAYS fill the black tank completely (even if it means standing at the toilet, filling the bowl repeatedly until the toilet "burps and gurgles)... That will help you get all the solids out of the tank...

If you're at a full hookup site, consider filling the black tank, dumping the contents, then with the dump valve open, connect the black tank flush and let it run for some time.... An hour may seem like a long, LONG time, but if you check and there's still "brown bits" (often called "sewer trout") then there's still "smell in the tank as well as the fish".... Keep flushing.

I think that if you always start with a relatively clean (not just empty) black tank, turn off the roof vent fan before flushing, leave the roof vent closed between uses if possible and make sure the toilet is in "working condition without the above issues" you should be able to eliminate most of your black tank odor problems.

It's not really about what brand "odor mask" you buy, it's more about having a tank that doesn't emit so much odor the mask can't possibly keep up with the job.....