Figure a STARTING battery is good for a dozen draw downs to 60% remaining capacity without really harming it. Starting batteries contain less antimony, so they require less watering, they self discharge less, so they corrode the terminals less. As few times as you use the cycling capability of your rig's battery a group 24 or 27 engine starting battery from Costco is my choice. They also have a very attractive warranty, especially the free replacement period.
All this gets tossed out the back window if you suddenly get an itch to go wilderness camping. Even so I have seen lots of people camping who limit their overnight discharges to 80% of capacity (20% discharged), and they get away with murder.
So it isn't just about battery cost. Purchasing two conversion terminals will switch a post battery over to stud connections like your RV battery uses. You'll love the less water needed and less corrosion part.
Whoa! Here comes all the screaming and moaning. I can see it now :)