Old Trailer? suggested I post this here?

I hate to say it but you probably won't have any luck securing a manual. From your post I gather that you are already aware of this, but your best bet is to go through the entire trailer and make a list of every device by name and model number. If you can't find a name and number then take a picture and post it here and someone can probably help.

Once you have your list you can start searching online or ask here and we can try to point you to resources to download the manuals.