I agree with Rich above, and here is my take. Your battery is 6 years old and has gone dead. It is probably done for and needs replacing. What type of camping you do is the best indicator of what type and how many batteries you should get and use as replacement. Are you always at a park with hookups? Are you always "boondocking" and rarely plugged into electric? 50/50, 90/10, or 25/75? If mostly plugged in, then you can get away with one battery. Watch this... https://youtu.be/WFPcV9_1frE
I'm not sure about your 2018, but BT Cruisers of past are wired as such to be able to provide charge to the "house" battery(s). 1) From the vehicle's chassis engine/battery. A large wire connects the engine battery positive to a constant solenoid. The other side of the solenoid connects to the positive of the house battery. When the engine is running the solenoid connects them and charge is given to the house battery from the engine alternator and charging system just as it is to the chassis battery. 2) The onboard converter also is a battery charger. The converter takes the 120v AC electricity from shore or produced by the generator and converts it to 12v power. When connected to shore power or the generator is on, the converter should be charging the house battery. Not all RVs are the same, and some are wired so the converter will charge the battery no matter what position the disconnect is in, but some are wired so that this will only work with the disconnect switched to ON.
I would watch that YouTube video and then keep watching and learning there. The more you learn and can do yourself the more $ you can save. If that is not a concern, then you can pay away the problems as they come. Best to you either way.