I don't know about the bigger trucks, but w/ vans ya can make out ok, not great but ok, if ya realize one VERY important thing. Do NOT buy a new vehicle for this venture. You'll end up working 2 weeks a month just to pay for your vehicle & maybe even more if ya have a slow month.
I would strongly advise keeping your van & insurance payment under $500/month. Keep it even lower if you can. It also might not be a bad idea to see how long your company will allow you to keep your van. Most companies allow em up to about 6 model years or so. Ask if you can grandfather your van in (like if you buy a 2002 van, will ya be able to use it after 2008 if you can show you've kept it up). If you can, find the best van you can around the 2002 model year. If you can't, purchasing a 2003 or 2004 might be the best way to go.
In this business, like a lot of em, it's not just about what you can make, but more about what you'll put in your pocket after all your expenses.
Good luck to ya,
Danny