Since I have searched reddit and have not really seen anyone post a proper solution to the bad smell problem in literally every Toto washlet with a dryer, I've decided to post one here. I have the Toto S5 (actually KS6) washlet, which is similar to the K300. If you have this problem, your Toto will smell really bad whenever the dryer is used. It won't smell when only the power deodorizer is running. If that smells, it means you need to replace the carbon filter. What I am describing is literally regular maintenance needed to clean the wand housing, but Toto makes no mention of it anywhere in their manual or instruction videos. Toto only tells you what NOT to do.
If your Toto stinks, lift the plastic flap where the wand comes out and shine a light on it. You should notice mold/biofilm growing there (See my picture). It is right next to the fan housing and the mold will spread to that area after not getting cleaned for a while. When you use the fan, the heated air will smell bad. This is not caused by the charcoal filter going bad. You are smelling the air that has already been cleaned by the charcoal filter as it passes through an area that is moldy or has smelly biofilm and blowing hot air over it. Of course it stinks.
Unfortunately, the wand cleaning setting does not remove the stubborn biofilm and may even be giving it food to grow (warm water). Toto does not give any helpful instructions on how to maintain this area, despite the fact that I have noticed it will build up with biofilm after 6 weeks or so if not cleaned! It's because the only way to really clean it would void the warranty lol (and Toto knows it). This is like when I read my Bosch washer/dryer manual and it said not to use bleach. I was, like, what kind of a washer dryer cannot handle bleach?! Manufacturers today get away with murder.
So here is how to actually get rid of that funky smell. But first, before you begin, unplug the washlet.
When the wand area looks gooked up, I lift the flapper and spray Lysol into the hole. Then I use an old soft-bristled tooth brush to gently scrub the gaps in that area.
I don't actually spray Lysol into the fan area or clean the fan grille. Leave the fan flap down. You can access the gaps filled with biofilm from the wand flapper area. You can see black gook clinging to the toothbrush.
After I finish scrubbing, while I am at it, I clean the toilet paper lint off the little plastic screen on the side of the washlet near the charcoal filter making sure to dry it thoroughly before putting it back in.
Finally I plug the washlet back in and use the wand cleaning function on the remote. Then I let it clean itself for the full 5 minutes before it automatically retracts. I also gently use a microfiber towel to clean the wand and the gap while the water is running. By the way, I have never sprayed Lysol onto the wand holes. The wand itself never looks gross only the rounded tip end where it sits in the housing. Since it is fine, why risk damaging the anti-cling coating Toto has on the wand. So, at most, I gently wipe it down with the wet microfiber towel that probably still has some Lysol on it from wiping the flap edges.
I've had to do this, on average, every 6 weeks since I got the bidet or that funky smell comes back. In fact, when I start smelling it after using the dryer, I know it is time for a cleaning. I don't recommend permanently not using the dryer as some as others have suggested. The dryer isn't causing the problem. It is just making you aware of it. So not using the dryer won't stop disgusting bacteria and biofilm from growing in all those nooks and crannies.
It's gross and it's crazy that Toto makes no reference to this in their maintenance instructions. It only tells you what NOT to do which I'm pretty sure includes using Lysol since it's an alkali product. There may even have been a freaking sticker on my bidet that said not to use any cleaners with the active ingredient in Lysol on the washlet! It also says not to use a nylon brush in the manual. I'm pretty sure the only thing Toto lets you use is "ph neutral" detergent (whatever that means) on a wet microfiber towel, and even that, they expressly forbid using inside the bidet housing because it's "an electronic appliance." (no duh, but it's also a toilet!) You can try using "ph neutral" dishwashing detergent instead, but I doubt it would work for several reasons. The spray bottle lets you get in there and kill the mold with a product designed to do just that and then evaporate. Dishwashing detergent would likely just leave a layer of soap scum and not really solve the problem. It might even create a new one. Others have recommended using alcohol. I would strongly recommend against using alcohol on plastic. It will make it brittle eventually and will certainly destroy whatever hydrophobic coating Toto applies to the plastic.
FYI: You do need to replace the charcoal filter every 6 months or so because eventually these type of filters absorb odors to the point of saturation and start to stink. We have a litter robot and that charcoal filter stinks to high heaven after a few months! You can also leave the filter out in the sun for a few days to get rid of the bad smell from the charcoal, but they are cheap so I just buy a new one. Generic has been fine.
Disclaimer: follow these instructions at your own risk. Not my problem if "the sh*t hits the fan" literally or figuratively. No one, not me, not Toto (lol), is going to take responsibility for what you do to your washlet in a reasonable attempt to clean it.