The best way to look after them is to keep them topped up with charge. The fewer and less deep the cycles, the longer they will last. They have no memory effect and therefore frequent charges are not detrimental to their performance. To maximise teir life, its best not to let their voltage drop below 12 volts however serious damage will not occur unless you let them go below 10 volts for any length of time, when sulfation occurs.
What kind of voltage drop will happen when I put a load on the battery?
Depends on the load.
how low will the voltage go with a 10 amp drain on the battery?
I would not expect anything more than a minor (0 - 0.5 volts) drop on a fully charged 30 Ah lead acid battery that is in reasonable condition.