Stop worrying about the roof leaking!
I have had two trailers prior to the one I am in now and never had a leak.
I did examine them once a year and cleaned and put a roof preservative on them.
Neither one had a slide, and for my take I wouldn't tow a trailer that has a slide.
I have a brand new 40 footer with slides that is on a permanent site and the slides will remain out, and the second I see any signs of a problem, I will install a roof over them.
There are way too many other things that can and will go wrong before any roof starts leaking, so make your purchase by the floor layout, not on what the rood is made out of.
90 percent of the trailers on the road are made out of junk and are put together by workers who have no pride in their work
Jack L