14.2V is about as good as you'll get at the battery. I work on sprinters with 150A alternators (standard is 120A), and even after sitting running for a couple hours, they'll still be around 14.2V. Even with a brand new loom, there is always a bit of voltage drop between the alternator and battery.

A bad battery will not pull the alternator voltage down, unless a cell has gone short circuit, which means it'll get warm very quickly, and gas lots. A bad battery is more likely to go high resistance, which means it's actually easier for the alternator to hold the regulated voltage.

Deep cycle batteries do have a different internal construction. Normal starting batteries are designed to output high currents, with the trade of being they're not as good at prolonged drains or being run flat for extended periods of time, whereas deep cycle are designed to handle lower currents, and have better recovery from being left discharged.