In for a penny, in for a pound. I wasn't happy with the frame sag. The doubled up frame was straight ahead of the dinette. The bumper dropped about 1.25" over the single beam section.
The shell is mainly supported at the front and rear ends only. Cantilever effect along the sides does not add much structural support. Some sag is normal and not a problem. That's my opinion anyway. Full disclosure: mine was worse because I overloaded the hitch then witnessed the frame flex too much so most of my permanent set was my own fault. Yes embarrassing to admit but if this is gonna add value to the community I may as well share the good, bad and ugly. (shrug)
My so called "improvement" was to both straighten and stiffen the frame. I did that by adding another 1.5x3" box beam splinted along side (most of the length of) the other wise single beam dinette area. Each "scabbed on" piece (48" each BTW) also extended forward for attachment. The curvature is evident in the first attached pic below.
Once the front was attached clamps were used to "spring" both towards each other. In addition a cantilever bracket was added behind the wheel wells at the dinette transition where double beam construction ended. The last two attached pix show that best. The frame is not perfectly straight now but it's better than when I took possession at age 16 so I'm ecstatic.
The cantilever brackets are key. They bears weight
where the cabinets end over double beams before the dinette section. And I'm not quite done. I also plan to add diagonal prop supports tied down to lower corners to make it even stiffer. Words to describe that are even harder but hopefully the pix help convey the idea until that's done. Hopefully tomorrow. (Had to see doctor for eye injury today so that needs rest first.)
I know this project presents both whacky problem and solution. Don't be shy with questions.