I am not positive, but I think the screen door of your Excella Motorhome is not the same size as the Airstream trailers, so you may have a hard time finding one. They still might be available from Jackson Center, but will be really costly.
The sliding windows and hardware are also somewhat special, but again, parts from Airstream may still be available. Search the motorhome section here for threads on windows, I bet someone else has had experience and may have posted it already. I never had the sliding windows out on my 310 or Argosy motorhomes when I owned them.
Reading/searching the motorhome threads here might be your best way to learn and get the courage for that first step. Best to you.