I filled in where the front window was and all of the now unused cutouts for the appliances in my Scamp
16' unit with pieces cut from the also removed shower stall. This gave me a flat piece I cut the the size of the hole I wanted to fill. I held it up the the hole and traced the cutout on it and carefully trimmed it to fit.
Then I beveled it at about a 10:1 slope on the inside edges and the same on the outside of the hole I wanted to fill.
I then backed up the outside with some thin boards along the cut lines and used heavy aluminum tape to hold it in place while I built up strips of fiberglass fabric and epoxy resin along the scarf.
This is what the outside looks like by the door.
And here is an out of focus picture of the inside.
Soon someone will post to tell you to use polyester resin which will work just fine as well. I used epoxy because I am more used to it and used it on aircraft repairs and construction.
The fiberglass I used was 6.5 oz. I bought off ebay (so was the 2 gallon kit of 1:1 epoxy.) I cut the fabric into 3" wide strips and this was the most irritating part of the process. I used strips of about 3" X 18" and overlayed them over the scarf.
Later I will lightly grind the outside and use filler to smooth the joint.
The flatter you can join the parts the less work you have to do later to smooth out the panel.
Others may offer a better solution, but I had the fiberglass scrap to work with and that is what I used.