We took pictures of most things as we removed them, and organized the pictures in a file on the computer. It's not a perfect system, but we can find things with a few minutes search. It did help us to remember when we had to take apart the rear to fix black tank flange that cracked and subsequent floor rot a few weeks ago (hope that never happens to anyone else. Ish)
We also didn't throw components away until we had most of the trailer rebuilt. We were able to reuse some things like the walls for templates. Sink was in good condition so we kept that (one of the few original things still in the trailer). We gave away things we didn't reuse that were still in good condition to others that were going for a more vintage look inside.