2 ajoining rooms at All Stars or Pop Century is the cheapest was to stay on property. We stayed at an off-site rental home once and found that the space and price were wonderful, but the commute was a pain and we really missed the magic (and perks) of staying on-site.
If you can go during free dining, do so!! Our family of 6 saved nearly $2200 this way when we went in 2007.
Another good idea is to order groceries from one of the delivery services and eat bfast in your room each day. Staying off site and cooking your own meals saves alot of $$ too, but again, you have to trade off the car rental cost, travel time and other downers of off-site living. And who wants to cook and do dishes on vacation?!
As for keeping everyone together, we just pretend that we are camp counselors.

We do count heads before we move from one place to another, and we use the buddy system: an older kid looks after a younger one, and DH and I look after them all. Having everyone wear a bright color -- traffic cone orange is our standard park uniform for the boys!

-- really helps.
We also "tag" the litte ones each morning, writing our cell phone numbers on the side of their shoes with a sharpie. We've never lost one of them, but our now 13 yr-old -- our very own absent-minded professor -- has gotten good at standing on park benches and waving until we find him.
Really, if you are already parenting 5 kids, you know how to do this. Just have a good plan, be organized, and have everyone pitch in as they are able. And have fun!