One thing I have found on Quickbooks (which is what I use) is you can break things down by "class". So each asset can have it's own class (which is just the asset number). So when I do invoicing or add fuel etc, I choose the truck number from the class drop down box and it will sort the profit and revenue by truck.
For my friends trucks we went with
http://www.boltsystem.com which is an online dispatch software. It is pretty good with fleet management. You can evaluate a lot of different things like lane profitability, truck, driver profitablitly etc.
They do requre a 7 truck minimum or least to pay for 7 trucks which comes out to about $150 a month.