Handling dynamics on a truck (handles badly) pulling a trailer (now handles worse) is not the same as a bus, a motorcycle or any car. Though the picture was likely an indication of a bit of exuberance, the fact remains that for an RV -- the worst-handling vehicle on ANY roadway -- prudence is first if safety is foremost. Braking distances, alone from 75 mph are huge. Few people have regualr experience stopping from this kind of speed with similar rigs, and misjudgment is almost to be taken for granted.
Virginia Code: Tables of Speed and Stopping Distances
And, while a brand-new trailer and truck are, hopefully, least-likely to suffer a failure it is still tempting fate. Under 70 is reasonable and prudent. And eats the miles while leaving a FAR larger cone-of-vision (peripheral vision) than above. Search out safety studies and make your own decisions. "Experience" is usually the lack thereof. Somehow I also doubt that the driver made the kind and timing of stops recommended (and proven) to avoid fatigue, the true problem. Highway hypnosis is nothing to sneeze at. Age and physical condition are no deterrent.
That said, I admire the choice of rig, hitch for intended use. I also recommend a complete set of scale weights to better dial in the rig for the best handling and long life. There is no better travel trailer for road-ability if one attends to all the details.