Theresa, I just may have some help for your (and others with slider windows) wet concerns.
By now I'm sure almost all of us have seen these (sometime tinted, sometime painted) automotive side window rain/window shades.... Well these would help your situation as well, you'll need to take the window width measurements of your trailer windows
and go to an auto parts store (even Canadian Tire) and find the size and shape that conforms to your window openings. These items stick on with (enclosed) double sided tape and it does provide a good (almost always) water tight seal. The only concern is that they are sold also with shades for the front windows
(of most vehicles) and their shape will probably render these parts useless at least to you. The parts you'd need are the shades used mainly for the rear windows
of cars. It might be a little spendy, but consider, you MAY be able to keep your windows open in most rains (maybe NOT wide open, but enough to let fresh air in or stale air out.