Here is what I did,I hope these pics can give the idea.
The Trill's have wooden window frames which make it very simple to attach and build sturdy shelves without drilling thru the walls but the same effect can be achieved with a minimum of drilling with the technique I used.
I just built 1x2 clear pine frames and screwed them together tightly. They are attached to the existing upper cabinets, wherever they are and then to just a few points around the outside perimeter.
On a Scamp I would probably have to drill through the wall and bolt through the shelf frame and then seal and cap the bolt heads.
Once fastened I covered the top and bottem with 1/4 " baltic birch plywood which is very strong yet lightweight.
I als ran power wiring thru the inner framework for new lights
under the shelves.
With only very few connections to the trailer if these are built carefully they are very strong.
I have more pictures if you want to see them.