You can build a bunk for a Northern Lite or any other brand. My bunk attaches to the ceiling in my Host and folds down, but there are other people have built frames that are above the dinette.
My Host was built in fall of 2005 and I've had it since May 2006. The slides and camper are still good. I do think it's important to make sure that you are level using slides. Obviously, the weight that is in the slide will impact how much they potentially would fail or put stress on the rest of the wall.
Any TC will be top heavy compared to a truck without one. If you want the least top heavy, get one without a basement.