There are no structural isomers of propane itself. By itself, isopropyl is nothing really--->just the same ol' n-propane. Isopropyl bonded as an alkyl group to something else is really just propane bonded to that something else via its second carbon (carbon #2). For example, that's why we say "isopropyl alcohol" as equivalent to "2-propanol". (or isopropanol)
As for the nomenclature, why would you name propane as 1-methylethane ? Propane itself can only exist as an n-alkane. Ethylethane is n-butane, but we just say n-butane.