Floor failure!!!

The way it is supposed to work for max emergency braking is that you set up your brake controller so that at its maximum adjusted output voltage (when the manual slide is fully on) the braking force is just short of lockup on dry pavement. That is the setup procedure the controller manufacturers recommend because that will give you the shortest stops. If you set the controller voltage higher and apply full brakes you will  lock up the wheels which will cause the braking force to go down because you're now skidding along on a patch of melting rubber so the friction coefficient is reduced. If you set the voltage lower then you are not going to get all the possible braking action if you ever need it. 

On many travel trailers, my rPod being one, you can't get enough braking action to lock up the wheels even at full voltage from the brake controller. The brakes just aren't powerful enough. Since I can't get there I have no idea how much more emergency stopping power my tires could give me before I start to skid.  If a trailer salesperson wants to call that a design "feature" ok I guess, but to me its a design weakness.   I'd prefer to have more brake capacity available and be able to adjust them with my brake controller to the level I want.

Yes, braking would act to apply a moment (torque) to the axle and frame on both sides. How much depends on how aggressively you're braking, how much weight is on your trailer wheels, how good your brakes are, and the coefficient of friction between the tires and the road. Typical dry pavement friction coefficient for street tires is about 0.7 just before lockup so theoretically you could get 3500 lb x 0.7/2 = 1225 lbs force at the contract patch on each side. With about a 14 inch moment arm (because I have a lift kit) that's 17150 in-lb, compared to about 12250 in-lb for the moment from the torsion axle at 1g.  In practice I can't lock up my trailer brakes (new and properly burned in) on dry pavement even with my brake controller on full boost so its going to be less than that. 

In any case, its pretty easy to see how the combination of some braking plus the vertical force on the frame from hitting a bump, plus the torque from the torsion axle from hitting its stop could combine to bend or break the frame at that spot. In future when I brake for a bump or pothole I'm going to try to be careful to release the brakes before the trailer actually rolls over the bump.  

No, the torque applied to the frame from the torsion axle and the brakes is in the same direction of rotation (counterclockwise when looking at the slide side). 

It's rumored that the HRE's have a 5200# axle. If true, are their frame's also of thicker steel?

I met a HRE owner that claimed his axle was a 5200#...