The charge controller / converter takes care of charging the batteries, many better quality c/c's are programmable for the battery type: Flooded, AGM, Gel.
If you find you have a problem with one battery being overcharged,
1. unplug the trailer from shore power,
2. let the batteries cool down,
3. check both batteries are identical
4. check battery voltage with Voltmeter
When working with batteries there are a few rules:
1. wear acid resistant gloves and safety glasses when handling questionable batteries
2. Never tip batteries over, unless they are sealed AGM type batteries
3. When connecting batteries start with positive cable then negative, when disconnecting negative first positive second.
Do get to your main question, it is very possible that the battery that's boiling is just defective and needs to be replaced. A fully charged battery should have 12.6 - 12.9 Volt when left disconnected for about 30 minutes to an hour, if it is less you have a brick.
2013 RAM 3500 Dually Longhorn
2014 Trilogy 3650RL, W/D, Onan 5500, 8000# Axles, IS, 960 Watt Solar, 6x6 Volt Battery Bank, Freedom SW 3012 Inv/Charger