We attached the lower cabinets to the walls with strips of aluminum riveted to the walls. The only cabinet base attached to the floor is the refrigerator cabinet. All the others "float" so they can flex with the trailer. The bed base is also fastened to the floor. Chris used Kreg screws to do that also. Upper cabinets are attached with Kreg screws at the base and a piano hinge across the top and riveted in place. He got that idea from another thread, and it works very well. All our screws and rivets are hidden inside the cabinets.
We used bass wood for hidden frames as it is very light weight, and used red alder for the visible portions of the cabinets. Red alder looks like new cherry, but doesn't darken like cherry and is half the weight. We are fortunate to live near a lumber yard that carries all kinds of unusual wood (not like Home Depot, or Lowes), so we were able to find it. Many people use birch, which also is light weight and looks beautiful.
Our thread is "Little Girl Refurb" in the 70's Sovereign
section, if you want more details of our cabinets.