I cannot tell from the picture what group size your old battery is. Most of the RV dealers use group 27 since they are a few dollars cheaper than a group 29 battery. Most Interstate batteries are marked with the group size on the battery.
You have picked a group 24 one size smaller than a group 27. I would go with the group 27.
The size of the trailer does not really determine the size of the battery only the load you will be pulling out of the battery. When you get home make sure you put the battery on a charger and charge it up good before you install it in your RV. Also make sure the battery connections are clean, shiny and tight when you install.