Stress crack in fiberglass

Frame flexing and or a broken weld. Call keystone and go from there.

make it a 1/16 inch hole and be sure it is BEYOND the end of the crack by 1/8 inch or so. The purpose of the hole is to spread the force creating the crack to such an extent that the crack will not propagate.
Fill the crack with some kind of epoxy and you won't see the hole (or it will be really difficult to see). It really works.

Very common problem, drill a hole where it ends, get yourself a piece of flat trim cut to size from bottom trim and top of crack butyl tape under trim over crack drill two holes a couple screws then caulk the edge of trim piece. It'll be just like thousands of other 5ers.

That's probably got something to do with the frame of the slide having a defective aluminum weld. Google it.

First thing may be to just drill a very small hole at the end of the crack to stop it from getting longer. My fw flexes a little little and has about an inch long crack right at the bend under the bedroom. My caulk/butyl tape also oozes a little along the front horizontals. My sons 35 ft FW is a little worse on the flexing and disturbing the caulk. There was a post a couple weeks ago from a lady with flex problem. His also makes creaking noise worse than mine.