Welcome to the forums!
Nice trailer!
I am not sure about the interior template "thingy".... all the Airstreams may be just ever so slightly different. If you want to replace existing ones (say in the bath) take the old ones out and use as a pattern. If you don't have a pattern, or something to follow you should make one from cardboard or some other material that might fit. Fit and prefit and double check. Then cut and fit the final piece. Check out the search tool on the blue toolbar and key in phrases that will give you the information you seek. I am sure there is info on fitting the interior panels.
Good luck!
Mrs. NorCal Bambi (traveling in S Tardis)