Chances are its a Lippert Frame which have a reputation for this. I have seen several. Make sure they sandblast the area first and get it re-welded and inspect the rest of it closely while you are at it. I would have them weld a 45 degree cross strut across that gap as well since this is obviously a stress point, and I would have them weld a plate across that crack. It will be stronger than just welding it shut. Do both sides. Do it before you drive it anywhere
Next time buy a Northwood, they make their own frames and they are stronger. Lance also does not use Lippert Frames, most other manufacturers do.