2001 Coachman Catalina 24 feet

I have Minnie Winnie 25ft but these are really similar as there isn't much room for design ideas.
Under our bed are the water pump, fresh water tank, and a lot of water lines.
At the outside is our main storage with also goes under the bed but there is no access under the bed to this area.
The side window does open, but the back window is emergency access only and therefore has no reason for a screen. If your back window has the ability to open, I would first look for where the emergency access is. I don't believe it would be any where in the front half of the RV as that is where the doors are. If you don't have an access window, I would be kind of worried.

Ours is a 1999 Coachmen Lepracaun --- the original owners manual & brochure is coupled with the Catalina series: the rear window opens as an emergency exit; not designed to be open for ventilation; no screens for it..... Hope this helps

Congrats on your purchase. Didn't know the Catalina was a class c until I went to the Coachmen site and discovered that in 2001 it was. The Catalina today is a travel trailer. The owners manual is available at the Coachmen site if you do not have it. http://www.coachmenrv.com/OwnerManuals.aspx It does not have anything about your storage under the bed. In my coachmen the storage under the bed is accessed from the outside storage doors. The rear window is normally the emergency exit, but you could add your own screening with velcro and then have the ventilation you want.