I checked if I could get the manuals as PDFs and for quite a lot of them I could quickly find the documents via Google or on the manufacturers product pages. I also had a few loose papers which I scanned via Scanner Pro and created the PDFs myself.
I uploaded all of them to my Evernote account, added headlines and tags and now I have fully indexed PDFs to search through, know exactly which model I own and can quickly find all manuals in case any blinking light catches my attention.
To be able to search in PDFs it’s necessary to have an Evernote Premium account, which is available for 5€ per month or 40€ per year. Moreover there are several campaigns where you can get a premium account, e.g. if you are a customer of Deutsche Telekom.