Current specifications for Kodiak Cubs do not include an 18 foot floor plan. The smallest is 21.5 feet. What is the model number?
If you have an inverter in your Cub it could draw the battery down quickly. Features and options on the Dutchmen Kodiak site do not include an inverter, but one could easily be added after market.
Next is the furnace fan. The furnace fan uses substantial current when it is running.
The refrigerator is another possibility. The options on the website don't list the refrigerator model. Most absorption refers use 12 volt for the control circuits, however the control circuits use very little 12 volt power. Run refer on propane to minimize 12 volt usage.
Some refers use 12 volts to heat the boiler in the absorption system. This type of refer could easily draw the batteries down overnight. It is unlikely you have one of these.
To know what is happening for sure, you could install a shunt type current monitor in the battery negative wire. You would be able to read current draw when various appliances are running.
Certainly battery capacity matters. My 2018 Cub came with a 70 amp hour battery. I quickly switched to two 100 amp hour AGM batteries. The 200 amp hour battery bank will last for 5 nights of dry camping in the summer. It lasts for Three nights of dry camping in freezing weather with the furnace running.
I wish you good luck and happy trails ahead!