we bought this same model and year last november 2018. i have not seen anything like that. in case you aren't aware i will tell you about the gearboxes on the slide motors. they have a history of leaking oil. a google search of keystone slide motors leaking will give you details.
i was not aware of the issue until last month when my routine roof inspection i do personally once a month at least showed 2 of my 3 had leaked a small dime size amount on top of slide roof. it causes the membrane to bubble up.
keystone's answer is to install small drip pans underneath gearboxes. im told they gearboxes stop leaking when they leak down to proper oil levels. it took about 9 months for mine to leak on slide top.
my spots were small, i cleaned them with dawn dishsoap and they appear ok because i caught it early.
some posts mention factory warranty covering membrane replacement and drip pans no charge. others mention factory only approving small patch on affected spot and drip pans.
the pans can be purchased by owner but i have a direct line to a representative that will ship drip pans free of charge. i talked to her on a monday and received 3 pans by wednesday. i can share her phone number.
sorry, not trying to hijack thread but very important info and i try to share with anyone with keystone and slides.
they may have factory installed drip pans on yours but doubtful.