gitrdun,
Try this tank from American Tank
http://www.auxtank.com/auxtank/searchDetail.asp
It's a 60 gallon combo made for use with a rolltop cover - rather expensive but may fit your needs. It's listed for a Ford but they should have one for a Chevy. I don't have any personal experience with this tank as I went the less expensive route with the Northern Tool 91 gallon combo. I did a little research and found a company that manufactures a wiring harness along with a 6 way switching valve that allowed me to plumb my tank in like it is factory with a switch on the dash plus it uses the factory fuel level gauge. Except for the shock of how much it costs to fill up both tanks - stock and auxiliary = 130 gallons = $350.00(+) I do like the 1500 mile range while towing which allows me to choose when and where I fill up.