I worked for a residential service company that did just about anything for years. They bought me a box-van (?) that was absolutely awesome. You know, it looks like the front of a van with a box body attached? Anyway, at first I was resistant to it because of many of the reasons that have been stated here. That changed VERY quickly once I got it.
My concern was ladders. They got me a three-section extension ladder that was only 11 feet long when stored. We put hooks on the inside right wall (all lined with nice plywood) and hung it there. Other ladders were hung below. Nice, easy to get to and protected from the sun/weather.
Since we were into just about everything, we needed LOTS of parts. We also did a lot of attic fans. The driver's side was lined with nice DEEP shelving, just like you would use indoors. It was about two feet deep and ran the entire length of the truck. I swear that I had just about anything on that truck, including two attic fans and an electric water heater with room to spare.
Pipe, ground rods and fluorescent tubes were stored in a rack suspended from the ceiling. With all of that inventory, there was still room to carry several varieties of light fixtures for property management jobs.
If I kept it neat, I even had a folding table that I could set up if I needed to take something apart or assemble, like a chandelier or celing fan. I just used a cheap folding card table and folding chair. It was so much easier (and pleasant) than doing work under the watchful eye of the customer.
I also outfitted it with a 1,500 watt power inverter so that I would have AC power if needed. I installed four keyless lampholders and 3-way switches for them. My boss got me some 12 volt "A" type lamps to fit in them.
Gas mileage and parking was definitely a problem. It was a great experiment, and probably could have worked out but finances resulted in the truck being sold.