Vin, propane is LPG. Butane is a form of LPG, too. LPG is Liquefied Petroleum Gas. Both are derived in a similar fashion -- either extracted from the flow of a natural gas field, or as a by-product of the refining process. As mentioned above, propane is liquefied at a lower temperature than butane. Butane also creates less pressure as the temperature increases, so that's why it's more effective for lower pressure applications like plastic cigarette lighters.
Apparently, they can be mixed, but I'm not aware of any butane being added to the propane I dispensed for over 8 years at my store.
The only chemical added to propane of which I'm aware is mercaptor (sp?), which is a foul-smelling chemical added to odorize the gas and make it possible to detect a leak by smelling it.
The primary reason LPG (Propane) is used rather than natural gas is because natural gas is much more difficult to liquefy, and therefore is generally piped to users. If there are no NG pipes in your area, it's too difficult to liquefy it, transport it in trucks, etc., pump it and store it in local tanks. LPG lends itself to that type of handling, so it's used most frequently in rural areas where there are no NG pipelines.
As for methanol used when purging a new tank, I have heard that Dry Gas (which I think is methanol or alcohol or whatever) can be added if you suspect there might be moisture in a tank. I never had to do it on the D.O.T tanks I sold, but I never sold a tank larger than 100 pound capacity (about 23 gallons). I've never seen a D.O.T. tank larger than 200 pound capacity -- not to say they don't exists, but the whole point of a D.O.T. (Department of Transportation) tank is that it's portable, so there's obviously a limit. The types of tanks most people have at their businesses or residences, and also in RVs if the tank is mounted to the chassis, are ASME (American Society of Mechanical Engineers) tanks. I have heard of these larger tanks having moisture issues. But, in the 8 years that I purged tanks, I never had to do anything except replace the air with propane vapor (which is what is accomplished by purging).