Have you checked out the Sloping Ceiling tutorial?

http://www.sweethome3d.com/slopingCeilingGuide.pdf

That should help.

I wonder if a parameter could be added to certain objects (such as box)
that allows to define SLANT
you have width lenght and dept plus elevation but you sould be able to do slant / or inclination either only on length or by choice on length or/and width such that you can have slanted object

i know that will be on heck of a development effort but it might be worthwhile