It depends on which propane system you install. If you go with a vapor system like the Powershot2000, you'd want a vapor tank. A BBQ tank is a vapor tank, but doesn't supply enough vapor to feed the system, but you could use two in tandem. I've got a 9.9 gallon Manchester tank mounted under the bed of my truck, and can go 2000-3000 miles between fill ups. Click here for pics of my setup.
If you go with a liquid system, like the Bully Dog, you'd need a different tank, but I don't have a lot of experience with that system.
If there's more I can help you with, let me know.