Jean....The front of a cabinet is called a "face frame" I did not put in a floor sheet first that already existed, this is in an motorhome, not in a van. As to attaching it to stud points. That would make it too dependent on the location of the studs which might not be located exactly where you need them to be. Instead you can run a cleat across the surface of the studs along the right and left side walls, secure that to the studs and then attach the face frame to that cleat.
In the case of the motorhome example in the photo above there were was no existing framing inside the walls because it was a molded fiberglass shell, a Sunrader motorhome. I had to remove the wall panels and epoxy in horizontal lengths of wood that was the thickness of the wall cavity to the shell itself. Then put the paneling back up. So my situation was somewhat unusual as many motorhomes have framing inside the wall cavity. But if you have framing studs already in the wall then the horizontal cleat can be attached to the studs on top of the paneling.
If it had been one of the newer composite paneled constructions that have no framing inside the panels and not suitable for attaching a cleat to those wall panels then I would have had take yet a different approach to create the rear bed with would have involved stick framing a box all the way around and then screwed that to the subfloor. Therefore the framing of the bed cabinet is not attached to the walls at any point it is only attached to the floor.
If you are going to renovate an existing motorhome you have to put your brain to work to figure out how you will secure things and still have it end up looking very nice as well as be strong. Sometimes that might mean removing the paneling and adding in more structure inside of the wall cavity. It is very difficult to anticipate what the best approach will be because of all the variations in the way motorhomes are put together. Plus of course the variations of what people want to have as their renovated design. This is why my first response to this thread was step number one is to figure out what you have in your motorhome in the way of internal structure and existing cabinets and then create a measured drawing of that structure before you attempt to do any renovations.
I have been building stuff all of my life, I began making my own furniture in my early 20s because I could not afford to buy any. The photo is of me when I was 30, back in 1981 working at Boeing in the "mockup" division part of a crew building a full scale model of an extended version of a Boeing 737. So while I am working on my own renovations I am not exactly new at this stuff, and it is not just a hobby or a one time only situation for me. My brain is trained to think this stuff through before I begin the project.