Coleman at that time was the only popup manufacturer that used fully boxed frames on all models, from the tiniest to the largest, so this trailer must have taken an awfully big hit to cause this kind of damage. ** Considering the degree of error described by the OP I wouldn't be coming to an internet forum for suggestions from complete strangers but have a qualified welder take a look and see what he suggests, the concern being whether the trailer as is currently even safe to tow in this condition.
** EDIT:
I've got a complete copy of Coleman brochures so I checked 1992 and found that back then the Destiny series did indeed have a C-channel frame while the American & Pioneer series did have the full box frame. You haven't told us which model you have but if it's a Royale, Cedar, or Roanoke with a C-channel frame then I wouldn't be surprised that it cracked. :(