Paul, I was able to mount shelves/upper cupboards using the existing window mounting screws, thus not needing to drill new holes. Our windows
are held in place with a “pinch rail” on this inside of the trailer, but perhaps your windows
use exterior screws. I used aluminum “L” stock from a hardware store, and some shelf supports from Home Depot to hold all this up. I cut the aluminum to the length of the window “pinch rail framing”, chalked the heads of the screws to transfer their location onto the aluminum stock, drilled the holes both in the stock and the shelf supports. If your Trillium
has exterior screws to hold the window in place, then you probably have wood on the interior of the trailer for the screws to anchor into. You could simply use shorts screws into this wood to attach the shelf brackets to. Best of luck with your project.
Dave & Paula