I studied petrol engineering, that does not, by any means, make me an expert. Propane tanks typically have a type of plastic based liner that is fused to the metal. I am not very sure how easy it is to remove that, as it has to be physically removed. Maybe after removing as much as you can, burn the hell out of the tank with several rounds of wood chips, or maybe coal. Something that gets it hot AF. I really don't know the chemical composition of the types of plastics that are used. It may not be necessary to remove the liner at all. As for left over petroleum, degreasers are all you need. Dawn dish soap and oven degreaser work fine. I know that tanks can be repurposed as a smoker or grill, I just have never looked much into it.