When I was highly active in consulting, one of the biggest issues I had was lack of any type of documentation of the systems, policies or procedures in use. Batch processes, schedulers, etc... happened in the background and noone ever noticed them unless something went wrong.
The only real way to fix this is to start including the time in each project to do the documentation for the project (even when the boss wants it yesterday!)
Then, when you have a problem with a system, you take the time to document as much of the system as possible as you fix it/investigate it.
It will take a long time, however, eventually you should start to feel confident that your network is actually fairly well documented, and that if a competent stranger walked in, he/she could start doing productive work relatively quickly.
Another alternative (ouch, expensive) is to hire some technical writers on a x-month/x-week contract to visit each of the areas and to sit with the people who run the areas and do the documentation for them. It will cost some good bucks (writers aren't cheap), and it will still eat into the time of your full-time techs, but, you will be able to deliver about 80-90% of your network documentation in a reasonably short time.