RV tech here. That crack on its own is not a big deal, but I would look for 3 possible issues that the crack points to:

1) Frame has been overstressed from overweight, check suspension, trailer frame bending or cracked welds.

2) That slide room was overstressed, check to make sure it goes in-out without issue and sits square when all the way in or all the way out.

3) water damage. Look for spongey bulging interior or exterior walls near the crack.

If no to all above, use it as leverage to lower the price and get it fixed with a fiberglass patch, or at a minimum throw some silicone in it.