Seamless indoor object (room) placement

I have searched here and on google but I don’t found anyone having my problem.

It is really simple: I build a indoor level out of 3 simple models. Each model is splitted up into at least 3 objects (floor, walls, ceilling - for texturing). That is working fine. BUT: If I try to build a big level out of these “modules” (or just a small floor with some modules) I always have these annoying gaps between them.
I know pressing v in the editor helps to position them right but it is not accurate enough. There are some little gaps.
Ingame these gaps flicker all the time I walk along this floor. It looks nasty. Somtimes I get them in a positon the rarely flicker. It’s not good but ok. At other places they won’t fit. I try to use correct coordinates but even with that these gaps are there.

How can I solve this? I think the problem is how I place the objects.
I use the standard Unity 5 Material with substances and the Default Material.

Tried Surface Snapping?

While dragging in the center using the
Translate Tool, you can hold Shift and
Control (Command on Mac) to snap the
object to the intersection of any
Collider. This makes precise
positioning of objects incredibly
fast.

Source: Unity - Manual: Position GameObjects