Propane is heavier than air, and heavier than mercaptan. That means, in a storage tank with a top-releasing valve, mercaptan would escape before the propane, which, upon releasing pressure, turns from liquid to gas. Storage tanks are not built to transfer propane in gaseous form, only liquid form. So, you shouldn't see these kinds of tanks distributing propane.
The normal distribution method of propane from one storage medium to another is to transfer the liquid - not gaseous - propane. So, if you were to fill a 1lb canister of propane from a 20lb tank, the method requires you to turn the 20lb tank upside down and hold it above the smaller canister, allowing the liquid propane to pour downward. But when you consume the propane - say, on a BBQ grill - you must use it upright. You don't want liquefied propane to feed your stove. First, because you won't smell the mercaptan if there is a leak, and second, because it's not in a proper state to burn, and you end up with incomplete combustion. This results in a more yellow flame with soot buildup, and carbon monoxide as a byproduct.
Now back to mercaptan: it itself is a gas and doesn't liquefy. That means, when you consume propane, you consume both gaseous propane and the mercaptan. When you pour propane, you pour only the propane - not the mercaptan (this isn't entirely true, there are additives to prevent this; but the illustration is valid). The large 100 gallon propane tanks are not meant to be used for consumption, only distribution, so, the tank and valve assembly draws the propane from the bottom of the tank by using the gaseous propane at the top of the tank to create pressure and push out the liquid propane at the bottom of the tank through valves that eventually come out at the top of the tank. Tanks are built to allow the transfer of propane as well as the mercaptan, so that your own tank gets its own supply of mercaptan.
So that is how the process of propane and mercaptan work, now we look to see where things can go wrong. The concept of mercaptan no longer smellable is called "fade". It can happen because of a leak, a reaction to the tank, or a reaction to something inside the tank. If there is a leak at the top of the tank, the propane and mercaptan will escape, and you'll know it - that's what it's designed to do. But over time, enough gas can escape, leaving only propane left, and then you would not smell it anymore. If there is an improper valve assembly that allows the improper distribution of transfer of propane (which must mix with the mercaptan at the top of the liquefied propane), that could cause not enough mercaptan to mix into the new container, or, too much which would leave nothing for later transfers. Finally, if the tank is improperly built, or an improper tank is used to store, then, the mercaptan can react with the walls of the tank.
What's more, mercaptan is heavier than air, so, it will tend to settle to the ground once it escapes. Once it gets into the ground, it stays in the ground - and continues to smell - even though the propane is not there. Therefore, you could smell the mercaptan, thinking there's a gas leak, when not only is there no gas leak, but the mercaptan is inside the soil or flooring. It is for this reason that if you smell "gas", you call a qualified person to detect and fix it, and also, to remediate the mercaptan residue.
You should also be aware that mercaptan is not always used, sometimes there are other chemicals that are used. Always, there should be a foul smell so that it can be detected. But depending on the chemical used, it may have different reacting properties with propane, the tank walls, or impurities inside the propane. Also, some chemicals are added to allow mercaptan to mix with the propane so as to mitigate a leak to allow escape and cause mercaptan fade. My knowledge here is limited, but I think the process is similar to how they add stabilizers to medicines so that you don't have to shake them, since the medicine becomes suspended in the medium.