Leg press, deadlifts, RDLs, lunges, leg extensions, leg curls, hack squats, split squats. All those will hit your legs, plus there variations. Do some sprints, run hills, stairs, those will hit your legs. Im sure there are more lifts I can't think of this second as well.
While natural gas occurs in nature as a mixture of methane and other gases, propane is actually a byproduct of both petroleum refining and natural gas processing.
Propane is a byproduct of the oil refining process and therefore fluctuates in price more so than natural gas. Since the USA is the Saudi Arabia of natural gas, it should be cheaper per BTU than propane.