When I was a parts guy at a RV dealer, the standard battery that was installed in all the RV's we sold was a group 24 deep cycle in a group 24 battery box. If requested, we could do a group 27 deep cycle battery in a group 27 battery box. I did a bit of experimenting and found out a group 31 battery would fit in a group 27 battery box.
For those people who pretty much went camping where plug-in electrical was available, the group 24 would usually suffice. But if they thought they might want to go to a "rustic" campground for a few days, I'd offer the group 31 battery setup as an "upgrade" option.
I'm out of the RV business now. But I told the dealer we are getting our new 2013 Eagle TT from, that I want the group 31 battery/ group 27 box combo on our new camper.
FAA A&P mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON