The sink and basin do not have S bends as such but the have the clear tube that loops down a bit before connecting to the pvc under the van. those clear tubes are always full of water so they are acting as S bends.
The main reason for the valve on only the shower drain is that it is at floor level and the sink and basin drain hole are about a meter from van floor height so way less chance of getting water to back flow to that height.
Well that is how I see it, I have spent a lot of time googling trying to work out the correct way and the way I am doing seems to be the most common. No one seems to no for sure though.
Cheers