Trying to make gameobjects disappearing smoothly

Hi guys,

I have a scene with different rooms and i would like to make a room fade out when the player is leaving it and fade in when entering it.

First, I thought about changing transparency of all materials of a room to make it fade In/Out but in order to make this work, every materials need to be transparent and this is causing rendering problems (see this : https://answers.unity.com/questions/129285/transparent-shader-objects-disappear-on-certain-ca.html ).
So I was looking to make a cutout shader but i dont understand how it can work as my materials are all opaque. Does someone have any idea how to do this ?

I'm also looking for other ideas to get the effect wanted

Thanks you in advance !
Corentin