i've had 'flexing' in 3 montanas. and the same crack on my first 5th wheel a Terry.
you didn't talk about the model or year of your trailer but that crack would be very normal, in my opinion, of the older models, that is is where the stress is/was when they started doing slide outs up front.
one of the guys above talked about the crack not being frame flexing, that is good. but that is as far as i can tell you.
i'd be interested in the model/year as we are writing.
good luck, not many dealers can correct these problems...maybe it is time to get a new one.
