Actually did this myself. Just got done actually. I have some pictures on another thread. You should check them out. Anyway, You need to be extremely careful on this stuff. People on here told me not to do it, but if you are careful with it you will be fine. Let me tell you what I did. I have a large propane tank that I used, that had been undergroung and not used for sometime by my church.
Well it had a fitting on the top that I removed with a monkey or pipe wrench. I let the thing air out for about 30 mins, while I got a water hose and drank some beer....Then I filled the thing up til it over flowed. Then I turned it over and drained it completely. Then I filled it again, using detergent as well with the water. Drained it again.....then here is the thing I did that I don't suggest but it work.
Me and one of my buddies waited a week and went back to check it out....we lite a match and the fumes were still barely there....So we grounded an electric fence box to the tank to get a constant spark. Plugged it in, went it his house for like 4 hours (to drink more beer) and came out unplugged it and filled it will water and stuff for a third time.....a week later we cut it open with a sawall and were fine.
May be a bit over kill, but it worked. Hope this helps you.