Driver fatigue often leads to "microbursts" of sleep.  It only takes a fraction of a second to drift off the road and once on the shoulder, anything can happen, including surprised overcorrections.  

Looking at this picture, is it possible that Furpod and others like him could possibly be right? That a truck-sized vehicle is optimal??

It's certainly a powerful argument for getting the biggest tow vehicle available, considering your other needs and the limits of your wallet.

From there, it's how fast, how much time to react, and ride it out. If everyone is safe, they are lucky. They just have insurance to deal with, and possibly a ticket if it's not mechanical failure or poor loading. The police will sort it out.

No obvious (to me) reason why this happened, but I am inclined to believe they lost control. There could be any number of reasons why. Was there a news story along with the picture? Do we know where this happened?

Now that I got a good look at the undercarriage, how difficult would it be to add a second axle between the fresh & gray water tanks?

Oh yeah. I see they had some kind of carrier on the rear hitch. It's not entirely clear to me what that is. I do see a fair amount of "turf" attached to the rear stabilizers.

This vehicle and the trailer appear to be locked together pretty tightly. I assume it's because they were using a WDH(?). 

We all are vulnerable to blowouts. Would it be better, if a blowout or similar unexpected emergency develops, to have only sway control on the basis that the connection with the trailer is pretty easily broken?? 

Blowout on left trailer tire. Trailer diverts to steep median, rolls and takes vehicle with it.

Call insurance.

fred