First off, the CHASSIS battery is the term used for the one that starts the engine and powers the functions of the driving part of the RV. The HOUSE battery is the term for the one that operates the living portions of the RV, the water pump, thermostat, water heater, refrigerator, etc. Was the RV hooked to shore power in between uses? Next you'll need to determine if the converter/charger that is powered by shore power charges both battery groups. To do this, after the batteries have been at rest for a few hours, take a voltage reading with a multimeter of each battery. Then plug in the shore cord and repeat the readings. A good battery should read over 12.7 v charged up. When being charged, the voltage should read over 13.4 v. If you don't see readings in those ranges, the converter/charger isn't charging both battery groups. You could also repeat the readings while the engine is running, see if the alternator charges both groups.
The chassis battery will be drained over time due to 'parasitic loads' like the engine computer (ECU) the radio memory and clock, etc. The house battery will be depleted by weak loads on it like the thermostat, LP gas detector, electric steps, etc. If you have battery isolation switches, typically near the entry door, while they suggest they cut all power to the batteries, they seldom do. If you park it for periods of time not plugged in, install disconnect switches that 100%. There are many styles:
Either those type of switches or a solar panel on the roof.