Deep cycle batteries are designed to be discharged all the way or close to that and then recharged. Using 1 in a single battery or dual battery vehicle without a charge indicator switch will shorten its life. Due to it constantly being recharged.
If you own a fishing boat with a trolling motor, the trolling battery should be deep cycle,, the cranking battery will be marine graded, which is really just vibration resistant.
I hope I answered your question, but thats what the guys from interstate battery told us 20 yrs ago at the truck shop I worked at.