For number 6, I assume that's mainly determined by where the steering column is in relation to the front of the chassis (for a Class A motorhome), which in turn is determined by the design and setup of the steering gear. The alternative I guess would be to have the windshield set back from the front of the vehicle, which maybe they think would look weird and might make the framing of the house part more complicated.
Having some space between you and the front of the vehicle is good from a crash safety standpoint, particularly if it's nicely engineered to crush on impact and absorb energy. I remember reading once about the length of the crush zone having a mathematical relationship to how fast a collision you could walk away from, based on the maximum deceleration the human body could withstand.
In a class C motorhome, of course, the size of the dashboard is entirely determined by the chassis maker.
My Honda Fit has about a two foot deep dashboard to the bottom of the windshield, for what it's worth, I assume there in an attempt to make a more aerodynamic brick shape.
