Years ago a mechanic friend of mine intentionally wanted to seize a Datsun engine by running it without oil (a long story). Took over 15 minutes, a lot longer than he expected. As a 16 yr old kid I inadverdently backed my Mom's '70 Mercury wagon off the ramps without oil while doing a lube job, probably about 20 seconds or so. No obvious harm at all. And through cumulative dry starts over an engine's life, it essentially runs many minutes without oil.
A deep cycle battery can provide a surge when needed, but nothing like the surge a car battery can. A deep cycle battery is also designed to be deeply discharged over and over again (something that would ruin a car battery very quickly). To accomplish this, a deep cycle battery uses thicker plates.
A RV battery designed for lower power and deep cycling will last longer and perform better than an engine starting battery. The best deep cycle batteries will last three to four times longer than a starting RV battery under the same conditions. Deep cycle battery capacity is measured in AH (Amp-Hours), meaning the total number of amps of power it can deliver over a 20 hour period.