Hard to tell without knowing the outside - if the outer walls are vertical, I would go with straight walls and add triangles for the support structure.

If wall/roof indents - that the windows protrude - it will require some measuring, exporting/importing/turning of .obj files to get it right.

As for the sloping ceiling, that may actually not be too hard if you draw a wall like this _/\_, export it to .obj, import and rotate it - and then stick the ceiling/roof on a separate level.

ok