More is being said about the frame rails, and I see no problem for years to come. I've removed 5th wheels on older trucks, and was horrified what was underneath them. Huge rust "tumors" that I can't believe still held. In some cases, it was the 5th wheel holding it together. I've seen lots of double frames in the dump truck business, and I don't see a problem here. Besides, you probably aren't going for max weight with a unit like this, like a rock hauler endures on every load.