Some have said here, they can go for days between dumps. The length of time between dumps really depends upon 2 things: (1) The size of your tanks, and (2) Your usage.
Of course, the smaller the tanks, the more often they have to be dumped. I have 3 tanks, 2 gray and 1 black. All 3 are 30 gallons. We do not use campground bath-shower houses at all. I have found, we can go 2 days (comfortably) on one tank of fresh water (42 gallons) and dump all three tanks every other day.
If doing so every other day, the tanks are never completely full, and if we do not have sewer hook-up, I can dump all 3 tanks and not overfill my 32 gallon Barker Blue Tote. By virtue of having to dump at least every other day, there's really no time for odors to form.
I do one other trick (all the time) when draining my tanks. I always drain the black tank first, then back-flush the shower grey water into the black tank and then dump the black tank again. Only the shower grey water drain is "T-ed" into the black drain. I can usually do this 2 or 3 times before the grey water is pretty much used up, and not enough to push itself back into the black tank. (depends upon how many showers we've taken). Each time the "back flush" occurs, more "nasties" are pulled out of the black tank. I have a clear section (Flush King) with the 3rd valve to make back flushing possible and can also see what's really coming out of the tank.
For those who never use a clear section attached to your dump valves, and have never attempted a back-flush, if you did, I think you'll be really surprised how much "nasties" are left behind after only a single dump. It's THIS accumulation which remains in the tank, over time, the really begins to smell.
If you get rid of "it" all to begin with, there's really no smells.
I'm a firm believer in using the grey water to back-flush into the black tank. It re-purposes the grey water, cleans the tank, and prevents using fresh water to rinse your tanks so much (especially in areas where water is scarce and precious). It just makes better sense to re-use shower water this way.
If I follow this procedure, my 32 gallon Barker Blue Tote never overfills, spilling "poo" on the ground and tanks don't smell.
Here again, the size of your tanks and the way you use your camper will determine how often you dump. How you dump will determine how clean in the inside of your tanks are. How clean the inside of your tanks are will drive what kind of additional "additives" you'll need for the tanks. This is way, simple dishwashing liquid and a bit of bleach every now and then, with clean water in the tanks and a road trip to slosh it all around is about all that's really needed to keep the tanks fresh.
