My sole battery is located under the entry steps, also. It is a 12 volt battery, and the original one sold with my 2013 Thor 31F. I've looked into its cells using a small LED flashlight and added distilled water until it came up to the full mark in each cell.
Can you not open the plastic rails that seal the cells, three on one side and three on the other? These plastic rails usually have to be pried up with a screw driver or something else pretty stiff. Be aware that the battery acid is potent, eating holes in fabric and harming skin and especially eyes. Wear safety glasses and chemically resistant gloves if you are cautious and far from access to water to wash off any splashed acid.
Charging coach batteries is usually handled by the shore power controller. I'm not sure if you could use a stand alone charger. Others will have to advise you on that matter.