I had read about the dreaded crack and last year Cupcake got one. Ike drilled a tiny hole at the end of the crack to keep it from splitting further. It seems to be stable at this point. Ike called (Inland???) and a closely matching caulk or maybe some kind of plastic filler was suggested. We haven't felt pressed to do a repair because it's not big enough to be a focal point. Plus trying to fix it might turn out worse than the crack itself. I guess the length & width of the crack would dictate how one might proceed.
Plus our ride is a lot smoother since we have a heavier tow vehicle now, new trailer tires and new shocks should be arriving ...actually later today as it's after midnight now. I have little doubt that vibration and shaking could cause that problem and a whole lot more.
Inland (Inlandrv.com) has a toll free # though it's difficult to get a non-busy signal at times.
good luck and let us know what you find out,
sue and ike
'75 Tradewind - 25'
2004 GMC Yukon XL 2500
Michigan - "middle of the mitt"