Check the wiring. With four batteries, you likely have two parallel banks of 2 batteries in series for a 12v system. It is treated as one big battery as far as the RV, converter, and other such stuff is concerned in most cases.
Is the separation between the bubbling batteries and the quiet batteries by sides in the parallel? If so, it implies that there are differences in batteries between the serial strings.
Generally, RV usage does not need equalization charging because the batteries are properly exercised and properly charged over sufficient time by a good battery charger.
If you are concerned about your batteries, first, check the wiring to make sure they are all connected together properly. Make sure your converter charger maintainer is working properly.
To diagnose problems or to equalize cell charge, work on one battery at a time or maybe 2 6v in series as one big 12v. Note that equalizing batteries is hard on batteries because the intentional overcharge to fully charge weak cells will overcharge the healthy cells causing plate corrosion and other such things. Also note that an equalization charge needs careful monitoring and hazmat precautions.