Jim, with all due respect and I'm not arguing with you. However you answered your own question. When you flush the toilet, close the vent/fan. It's as simple as that. No odors will come out the toilet with a quick flush if the vent/fan is closed/off as the roof vent releases all pressure. It doesn't matter how full the tank is. When not flushing, the entire system is sealed (toilet) and the outlet valve is sealed... you're not gonna get any odors inside the rv.
Now if you hold open the flush valve, yeah, odors are gonna come up. But with a quick flush, you will get nothing or maybe just a quick whiff. Close the exhaust vent/fan and all will be well.
Also, if you're in the "process" of using the toilet, the current deposit may provide some unpleasant odors. I have found that if I use my heel to flush as I go.... no odors.
With the roof vent, it is designed to vent the vapors and gas build up in the tank. I have never in all my years smelled odors coming from it.....even when I'm up on the roof. If you smell odors outside, it will be coming from a leak or an unsealed sewer opening.
I understand what you are saying and I'm sure you firmly believe in what you're saying.....but, do an experiment and prove it to yourself. Next time before going camping, drain your tank and put several gallons of water in along with a cup full of some sort or soap (laundry or dish) and then drive to your destination. Once there, empty the tank and add a few more gallons of fresh water before using the toilet. Every time you do use the toilet, add water to the bowl prior to flushing and add a little more water to the bowl after flushing.
Try it. What do you have to lose.
Ron