Unfortunately, I was happy too early. The groups are a huge problem and I need them because with so many objects I lose track without the groups. But even if I have no groups, still dozens of objects cut out a corner where they should not. And to edit the staircasecutoutshape for all these objects can not be a solution, because every time I change the interior design of the houseboat I would have to adjust the cutouts again.
I am now trying to find a solution with a framework. This is basically a 1000x1000 meter square, which has the cutout for the hull in the middle. But unfortunately I have problems here too, this time with the texture. As I can see, a length of the texture is stretched to the length of the object. Can I change that, so that the texture is lined up in all directions next to each other, as if it is on the ground?