I guess you're talking about a Coachman Class C RV, not the little fellow with a pot of gold if you catch him. (it is
St. Paddy's Day) Your RV has two battery systems, the chassis system, probably a battery under the hood, is to power your engine and driving needs, (wipers, HVAC, driving lights, etc.) The House system is a separate battery that powers the needs of the living area. Lights, furnace, refrigerator, water pump, etc.) That's probably the battery you found in the compartment. The two systems share a common ground. They may connect together when a charging voltage is present, either from the engine alternator or when plugged into shore power (120 v) The shore power should connect to a converter/charger that converts the 120 c AC to 12 v DC for the house functions. Not only does the converter/charger power the house needs, it charges the house battery. If the RV is so equipped, that charging voltage also charges the chassis battery. You may have a 'Boost' switch on the dashboard that connects the two battery systems when extra energy is needed to start the engine. It's like push button jumper cables.
To determine if the two systems connect together, use a multimeter and take voltage readings of both batteries. (A fully charged battery will read 12.7 volts) When the engine is running, take 2 new readings of the batteries. If they are charging, it should read over 13.4 v to provide charging. If you don't find the higher voltage in each battery, then they are not tied together. There are things that can be added to connect the batteries for charging but leave them separate so over use of one doesn't leave you dead also on the other.