I installed new batteries on our 2011 Sunseeker model 3100 this morning. I put in Duracell BCI size 31 batteries. They fit fine. There was one problem. The hold down strap's clamp is on the side, between the battery and the battery box sidewall. It was hard to reach with the original size 27's, and impossible with the size 31's. I had a webbing belt in my stash that was just like the hold down strap. I put one end through the hold down strap clamp and put the buckle on top of the batteries where it's accessible.
The Interstate size 27's that I removed, which weren't the original ones, and all the other size 27's I looked at, have lifting straps that go through lugs on the ends of the battery cases. The Duracell size 31's have a built-in lifting handle, and don't have the lugs on the end. (The original batteries had built-in lifting handles.) Although the charts for BCI battery sizes give the 31 as 0.9 inches longer than the 27, without the lifting strap lugs, there's not much difference. The lugs are not included in the BCI case dimensions.
Whatever you buy, be sure you get true deep cycle batteries, not starting batteries. Some, like Duracell, Interstate and NAPA, have both threaded stud terminals and standard posts so they can be used as a starting battery. Battery vendors have BCI size 31 starting service batteries that give high amps for short periods and are great for trucks, but are not what you need for an RV that uses low amps for long periods.
I don't know if the Duracell brand is any better or worse than other brands that are readily available, but the price was good at Batteries Plus. If you read the many posts about batteries, it seems that Trojan brand is highly recommended, but the prices I found are double what I paid. For our use, it's not worth the difference. As with conventional flooded lead acid versus AGM versus lithium, no battery is best for everyone.
2011 Sunseeker 3100
2016 Ford Edge Toad