You’re past the 3 yr frame warranty but doesn’t hurt to call and ask. Even if they won’t cover it, they could tell you shops in your area that do their frame flex tests and warranty work.
Link to Keystone Customer Service:
https://www.keystonerv.com/customer-service
There are posts on here about it - try searching for, frame flex. ChuckS has a post and video re details on the issue and doing the measurements yourself to see if that’s what it might be.
FWIW, a service rep showed me where Montanas have a screw in the lower corner of the skin/seal of the slide where they see cracks propagate from.