Like said already, you need to be organized and so does your van. I do a lot of service work and try to keep most stuff on my truck, your never going to have everything though. Just don't keep cases of everything, service vans don't need 50 4inch square deeps or 100 1/2 through 1inch EMT connectors. I'm lucky in my area I'm never more than 15 minutes from a supply house for the most part or the depot if need be.
I use to keep to much stuff in my van, making it a mess and hard to find stuff. I was working in a government facility where you went over a scale going in and seeing my 8600lb gvw van coming up at 9400lbs I decided to trim it down.