Spliting one large dropdown navigation into different parts in WP admin

Ok so here is the deal I am building a large municipal website that has about 500 pages split almost equally across 5 different sections.

Right now all the pages controlled in one vertical drop down menu via the WP menu admin. So the actual menu admin area is a nightmare to work in. If I add a new page it appears at the bottom of the navigation list and takes forever to drag and drop into the right position.

I am wondering if it would be possible to create 5 different menus (one for each section) then combine these on the template into one drop vertical drop down. This would reduce the length of each menu in the admin screen by a factor of 5. Has anyone done this if so can you point me in the right direction?

Alternatively does anyone know of a hack or plug-in that allows the parent items in the menu admin screen to toggle open or closed?

There has got to be a better way to manage large menus in wordpress am I just missing something here?

Please advise