Keep in mind that wind force goes up with the square of speed.
Yes, 1/2 * rho * v^2 * Cd * A, the hard part is that pesky Cd term. It seems to be .567 from the front (based on fuel economy) but that's with Scooby in front, so the calculations might be off. Still, if we use that as the coefficient of drag, and use the side area, a 40 mph crosswind generates 712.64 lbs of force! To compare, I'm estimating the force on the front of the trailer at 75 mph to be at 313 lbs. After looking more carefully at this I'm definitely going to be extra careful on this trip, particularly since we will be going through canyons where the wind can do stupid things. Luckily, we will be leaving bright and early in the morning and arriving at noon, so hopefully the wind will stay down until then.
There's a video of this guys whole build that is truly inspiring. He's a champion whitewater slalom kayaker who built this with a buddy over a couple of months as a mobile home to take on the Kayak circuit in Europe. He built this on a ford transit frame that is quite popular in the U.K.
I'm gonna have to check that out - I like the Transit vans, though I probably wouldn't tear up a brand new one. Once they are all a few years old it might be fair game to play with

The sad tale of the Penguino I.
Very foreboding to read indeed - looks like you got some warning shimmies before things went south though? I'm hoping that's the case - I haven't had mine shake yet, but if it does I'm going to pull over until the wind is gone. And if I ever replace the axle in that thing I'm going with an 8' wide one!