Originally Posted by VinceU
Do not purchase "Marine/RV battery". Only deep cycle battery will go the distance you want. A 27 is somewhat larger than 24 series and much more capacity. Go online and get specs including height. Check your box/space and see what will fit before you decide.
DEAD wrong. A marine RV battery can be either TRUE deep cycle OR dual use.
Example DEKA Marine master line has BOTH a DP27 and a DC27...dualpurpose v. deepcycle. http://www.dekabatteries.com/assets/base/0194.pdf
The DC series is an EXCELLENT and WELL BUILT choice in a deep cycle battery.
To the OP...as a practical matter ANY group 27 is going to perform longer to a 50% charge than any similar group 24. Most have around 90 amphours vs. the typical 70-75 on a group24.
I do think that even with your low daily use you are going to be using more than 45amp hours over 3 days so regardless of what group size you get...the cycling below a 50% charge will kill your battery quite a bit earlier than stickint to the 50% rule.
I assume you can't swing another battery in place so I will simply suggest that you get the one battery I know which is a bit more impervious to cycling below 50% and that is the Odyssey TPPL battery (thin plate pure lead) which is a special type of AGM battery rather than a wet cell. They can be discharged down 80% and still deliver 400 cycles! They also have MUCH greater current inrush and outrush capability to you are not limited to a 20% charging rate per hour. All at a price of course!
The good news is that SEARS carries these batteries (some of them) under the Diehard PLATINUM label at about a 33% discount. You may have to do some measuring to see what will fit in your coach but the largest that will fit is what you should get. Figure between $200 and $250 for one of these if you go this route.
Otherwise would recommend the Marine Master DC24 or DC27 or the Trojan 24 or 27 TMX series in a premium wet cell.