We would need to see a photo to say for sure, but it sounds like you've got the usual Ensolite on the bottom, and something modified on the upper half (normally they are 100% Ensolite).
As far as the seams go, I'm guessing you have something that looks like a grooved, plastic "rail" on the walls running along behind your seams. From the factory, there was a plastic, T-shaped molding strip that "clicked" into that and then the top of the T covered the seam (the top is something like 3/4" across).
As far as I know, no-one has sourced that exact molding, although people have modified a product known as "T-molding" to make it work. I think that the center "leg" on the currently available T-molding is longer than what was originally on the Trilliums.
This photo will give you the idea of the T molding - just pretend the flat wooden panels are the Ensolite walls:
Some T-molding can be found at www.t-molding.com
(smooth molding); but maybe the original profile exists if one were to search "t molding" on Google (and if one had a piece of the original or the measurements to compare to - I don't).