be sure to look at the test date on the tank, if it doesn't have a current hydro test date, you can't get it filled untill it has been tested.
underbody LP tanks do not have a testdate, only stand up cylinders do. However if you install an underbody tank and it is an older model, you might have to put a new 80% valve in. Older tanks only had a bleeding valve which you open when filling, as soon as you see white vapor exit the tank is at 80% and your supposed to stop filling. The new valves will automaticly shut off at 80%.