There isn't a vent in the toilet. The vent is normally in the tank, which you are discarding. The pupose of the vent isn't to keep gases from coming back, it's to relieve pressure in the tank (and piping) to allow water to flow in.
RV toilets don't use enough water to push the solids down the pipe, like a household toilet. That's why most of them have a spray nozzle nearby. If they did use the same amount of water, it would fill up most black tanks pretty quickly.
And your final question is true, most people keep the black tank valve shut until the tank is almost full.
If I was you, I would put in a regular toilet, I think it will work better. When you go on the road, replace it.