Neither manufacturer names the other. One publishes what the device requires, the other publishes what the battery does, and the two are compared here, including wherever they do not meet.
Winnebago specify “2 deep-cycle Group 31 batteries AGM maintenance-free” for this floorplan. Interstate state this pack is “Group Size 31T” and an AGM. BOTH ARE NAMING THE SAME BCI GROUP, which is a dimensional standard — 330 x 173 x 240 mm — rather than a measurement either of them took. A pack that is a Group 31 fits a Group 31 space by definition of the standard, so the size question here is settled rather than estimated. Interstate also describe it as a 2-in-1 doing cranking and deep-cycle duty, so it is a candidate for a starting battery elsewhere as well, and Winnebago ask for deep-cycle here.
Source: winnebago.com/models/product/specifications?ProductID=PROD754
Source: interstatebatteries.com/products/31-agm7
Only the house bank, and Winnebago fit TWO of them — this is not the chassis starting battery — check yours before buying.
Two of these.
Sources do not establish: which terminal type the van expects. Group 31 comes as 31T with top posts and 31A with threaded studs, and the two share an envelope — so a pack of the right SIZE can still have the wrong POSTS for the cabling. Winnebago do not state which their trays are wired for, and this pack is a 31T. Check the terminals before buying.
Source: interstatebatteries.com/products/31-agm7