It's not the 1/3-2/3 length ratio that makes the difference, it's the ratio of track width to axle-to-Hitch distance. A lot of cheaper tent trailers were less than 1:1.5 ratio to get width without length, and swayed unmercifully even if loaded very nose-heavy. My homebuilt M/C tent trailer, built on this ratio, tows dead straight at 90mph with my two-wheeler. My old Layton is not quite 2/3-1/3 length ratio, but is just shy of 1:2 track to draw length ratio. It's almost impossible to cause to wander unless the black water tank (at rear)is full, and the fresh (at front) is empty. But we're talking about 45 gals of slop, hung right at the back end. That would screw up any trailer's handling. There was a post on this thread that stated that the frame would need to be strengthened most of the way from the hitch to the axle
if it was lengthened. I agree that the greatest 'moment of bending' would be at the front of the body, as the section under the body is re-enforced by it. But, placing a piece of square tube (1/8" wall thickness) and same width as original frame, extending 1.5ft behind the front body line to 1.5ft in front of the extension, under the original and welded full length, should certainly add enough strength. Most of us, when modifying tongues, tend to 'Overbuild', where manufacturers tend to push to the limit.