If I remember correctly this picture is from a 2002 Adventurer. I am not sure when Winnebago downsized the battery compartment. I know my 2005 Voyage doesnt have this much room, so I put my starter battery in the compartment just aft of the steps.
I went to 2 golf cart batteries in a fifth wheel and the capacity was super and they lasted forever (years - wise) for a seasonal user. They were still going after 7 years when we sold it. The only issues was they were taller than the standard battery so they didn't necessarily fit in the RV battery compartment. I don't know the math, but I imagine the AGM is the preferred option now. Maybe golf carts even use AGMs today?
It's strictly a matter of physical space and the number of Am-hours you want available. The GC2 and GC2H 6v "golf car" batteries are a different shape than the typical 12v deep cycle, which will typically be BCI case size 24, 27 or 31. The 12v are usually longer and lower than the 6v GC family of sizes.
As kdbgoat says, the main advantage is that you can get a pair of true deep cycle 6v GC batteries for much less costs than true deep cycle 12v's with similar AH capacity. Often 40% less (prices vary by locale). Gold Car batteries are produced by all the major battery companies and in high volumes, so the pricing is better than 12v deep cycles, which is a niche market .
You can look at the case sizes online - find a BCI battery size chart. GC2 and GC2H (higher) batteries typically provide 210-230 AH per pair. A pair of Group 27 size 12v deep cycles would be about 210 Ah and a pair of Group 31 size 12v's about 230 AH.
I had an '02 Adventurer and replaced the 2-12's with 2-6s, worked great and used golf carts from Costco. They fit fine, no height problems and I could have actually fit two more in there like the photo shows. They're meant for a large discharge so will last longer in RV use. There is a ton of information in all the forums about this switchover.
In one example, we had 3 12V deep discharge batteries, originally. Folks suggested that 4 6V golf cart batteries would be much "better" so we tried that for about 6 years. The 6V batteries were quite a bit taller but they did fit... the frame was really close. Then one of the 6V batteries failed so we decided to go back to the 3 12V batteries. The key factor for us was the maintenance of the 6V batteries. Seemed like every month (or more) I had to add water to the 6V batteries and come to find out, we did not use the extra capacity... so we switched back and now, there's no maintenance... OTOH, we have about 2/3rds of the capacity if we ever need to run on battery power. In reality I think our 2000 watt inverter uses 300 watts just to be "on" so using the batteries for a/c power doesn't seem practical. With the inverter "off", the batteries could probably run the heater and a few lights over night. Much better for us to be on shore power...