The next step when the dealer doesn't fix it is the manufacturer.
My Dad got a new trailer last fall and several of the windows would not open. Turns out there was some dirt in the tracks that needed cleaning and then they worked fine. I don't know why yours don't work, but if it is because they won't slide, I think that is common and usually pretty easy for the owner to fix.. shower door shattering, something is hitting it or something is flexing to cause that. If you don't want glass, change it. We have a sliding plastic shower door, if it fails for some reason, likely just put a shower curtain in its place..