Propane Tank
Generally I tell my customers that if the trailer is not in use, turn the tanks off. If you don't have a leak it really should not matter, but it is best to be safe.
IF the tanks have been overfilled, there is a safety valve that will allow excess to bleed off. This can happen when the temp. rises during the day. It does not matter if the main valve is off or not, the safety valve will still vent and that might be why you smell it every now and then. When the level gets down where it should be, this will stop. The new OPD valves are supposed to prevent overfilling and usually they do.