Kev, I'm always amazed that you don't use chemicals. But I think perhaps the size of your rig has a lot to do with it too. More space for any stink to dissipate, you know what I mean?
We use TST chemicals. We boondock the majority of the time and tend to go at least a week without dumping. In our 24' rig, there's not much space for stink to go, other than our living quarters, which is why we use them.
Also found that the tank vent gadget thingy (oh what is that called?) that looks like a shark fin and goes on your roof, made a HUGE difference in getting rid of odors.
Be aware that none of the tank chems are "digesters" as your familiar with. Nothing digests in an RV black tank as it does in a septic system; that's why they're called "holding" tanks. The chemicals just mask stink.
What are shams are the cleaners that supposedly clean muck off the sides so your tank indicators work. These "cleaners" never, ever work.
Some people swear by the Geo Method (google it). It's a homemade combo of water tank softener and laundry detergent. I haven't been brave enough to try it. One of these days.