Many travel trailers are built upon a "deck-over" type frame construction. As the bare frame sits there, the entire length and width of the floor is totally flat, no "wheel wells" required.
Not only simplifies construction, but allows for slide rooms to be "flush floor" design.
If you build with wheel wells that rise up above the floors, you can have a lower frame, but your slide room now has to be built elevated to clear the wheel wells when retracted. This means you will have a step up into the slide out area inside. Some Jayco's and Lance are built this way.
