Do buck converters increase the amp hours of a battery?
You are kind of on the right track.

The amp-hour rating of the battery specifies how many hours you could pull a certain rated current from the battery. 

The buck converter to 5V won't change the amp-hour rating of the battery at all. But if you were to only pull half of the current from the battery then the battery will last twice as long.

So let's make an example here. As you stated you have a 12V 18AH battery. Let us consider, for sake of example, that your load at the stated 5V required 500mA (i.e. 0.5A) to run.

If you were to use a linear regulator to drop the battery 12V down to 5V the linear regulator would pull 0.5A from the battery and radiate (12V-5V)*0.5A = 3.5 Watts of energy as heat whilst delivering the 5V to the load. The 12V battery delivering the 0.5A would supply the regulator for 18AH/0.5A = 36 hours. The efficiency of such a system is EnergyOut / EnergyIn = (5V * 0.5A)/(12V * 0.5A) = 41.6% efficiency.

So if instead you used the buck switching regulator at 90% efficiency we can evaluate the battery life. The buck regulator operates as:

Energy Out = 0.9 * Energy In. 

With a load of 5V @ 0.5A the energy being delivered out of the buck regulator is 5V*0.5A = 2.5W. Applying the 90% efficiency formula the regulator input energy would be 2.5W/0.9 = 2.78W. This wattage at 12V corresponds to 2.78W/12V = 0.231A draw from the battery. At this lower current the battery would last 18AH/0.231A = 77.9 hours. 

As you can see the battery usage is hugely more efficient.