[JRTJH]

This is a diagram of the "basic" plumbing schematic for most "stick and brick" (S&B) homes. If you notice, the tub, sink and toilet all drain into one common drain. If the drain were to plug between the toilet and the vent stack, it would back up into the tub when flushed. It likely wouldn't back up into the sink because of the elevation difference, but the tub would not be so "lucky". The only thing that "might" (likely would not) prevent backflow into the tub would be the P trap (which will prevent gas odor from entering the tub drain, but not prevent effluent from backing up into the tub. With the toilet bowl water level higher than the tub floor, chances are it's going to have "brown water" if the drain plugs between the toilet and the drain stack.

Maybe it's not done this way in Australia, but this is the typical S&B plumbing in the US. As for RV's, once you get to looking, you'll find that in almost all two bath (bath in front with a half bath in the back) RV's drain the rear sink and toilet into a common tank. It may not be to "someone's liking" but it's the way the RV industry does it in almost all the larger RV's that I've seen. Keystone is not the only manufacturer to do this, virtually all of them follow similar practices.