Battery capacity: Says 12V18Ah/20Hr - how do I interpret that?
As @Felthry touched on in his comment, the 20hr part of your 1218Ah/20hr label tells you that the batteries' storage capacity was measured based on a 20-hour discharge (from fully charged, to fully discharged) rate.

All batteries have what's called "internal resistance" which means that the faster you charge/discharge the battery, the more energy is wasted as heat in a process very similar to ohmic heating of an ordinary resistor.

Because of this, battery capacities are often listed with a "discharge time" qualifier to give you an idea of what they're designed for; higher-end batteries, made for grid-storage applications (power substations &amp; the like) will often come with a chart that shows you the total discharge capacity at several different output amperages (discharge rates), but with less expensive batteries, we're expected to "be glad we got the data at all."

With that said, as long as you're not drawing more than 1.8A from your batteries, you can do your calculations based on a 36Ahx12V=432Wh (watt-hours) total capacity for the two parallel batteries. However, if your current exceeds 1.8A, then you'll have to expect a bit less total output, as more energy will be wasted heating up the cells.