Most RV's use Group 24 (size) batteries. The length and width of a Group 24 is the same as a 6-volt, but the 6-volt is taller by about an inch or so. A Group 27 battery is longer on the long side, but the same height as a Group 24. A pair 6-volts has almost identical total Amp Hour capacity as a pair of Group 27's.
Long story short: if you have a pair of Group 24's now, you can upgrade to Group 27's (if you have the room in length) or a pair of 6-volts (if you have the height) and either will gain you more capacity.
Some batteries are NOT the same as others. I have seen a set of "Group 24" sized batteries that were noticeably lighter than the ones I replaced them with. Your battery capacity is directly related to the lead content, and obviously some manufacturers are skimping.
Costco and Battery Systems seem to have the best prices/quality in the Tri Cities.
I can tell you from personal experience that (4) 6-volts, all LED lighting, and (2) large solar panels makes for a very nice dry-camping setup.
And by all means, be sure to use tools to tighten the battery terminals. I see a lot of batteries that live a very short life because the terminals were loose and the batteries were not charging well.