Hello & Welcome Sandy and Debbie:
I have a 2001 with a Bedroom Slide out which had the same type of crack which you mentioned.
Someone told me the cause was a poor weld on the Aluminum frame.
I do not know if that is true or not.
I had it repaired/patched and the crack returned.
I subsequently left it with WRV and they repaired the problem than patched and painted.
I also know of other Alpines with the same problem including a 2002.
I would suggest contacting WRV and see if they will repair the problem.