t is a classic stress crack. Some manufacturers cut the skin to prevent this type of crack and insert a piece of 'H' trim.
My '97 5er with a bed room slide started the infamous crack in the same place when it was about 3 years old.
My RV dealer gave me a 6" long piece of molding.
I used a thin radiac wheel and cut the skin on down to the edge molding.
I ran a bead of 35 year flexible caulk over the cut and used two small SS screws to hold it in place.
No issues in 15 years.
If you notice many 5ers of the '00s era have a short piece of molding installed at the factory in that location.