My converter fan runs only when the charger is delivering a substantial charge to a significantly discharged battery. At trickle charge output, the fan shuts off. I have solar on my rig, so when I plug into the generator to run 120 volt appliances (e.g. Microwave), the converter fan runs in the morning after overnight discharge, but not in the evening, because solar has the battery topped off. If you think you got screwed on your battery, bring it to an auto parts store and ask them to load test it. If it's bad, they'll tell you, and you should get a replacement from the RV dealer.
The parasitic loads include: CO/propane detector, stereo head unit and/or DVD entertainment center head unit. That should be pretty much it. While the CO/propane detector MUST be on at all times unless the battery is disconnected (for safety reasons), the fact that the entertainment center is a parasitic load is just poor design. Having the USB plugs live should cause no problems unless you load them.
All other items should turn off with various switches: You mentioned "lights."
I'm not clear if you mean entertainment system pilot lights or cabin lighting. Entertainment center pilot lights, clocks, and so on may well stay on.
Cabin overhead lighting, furnace, hot water heater, fridge, water pump and so on should all turn off definitively. If any of those items remains on after switching them off, I'm at a loss other than thinking they were wired to bypass their own switches. I'm assuming this is not the problem you are experiencing.
Several have suggested a battery disconnect
. If I recall, NoBo campers have these. Check either in the "pass thru" compartment (or any storage near the tongue) or in and around the battery box, and you may find one. If it doesn't have one, you can install one on the GROUND wire. Make sure that all ground wires connect to the switch so those sneaky parasitic loads are completely turned off.
Given you have a NoBo, I'm surprised that it doesn't have solar. Solar can easily keep up with the parasitic loads....it does on my rig. I have a single 12 volt group 24 battery and a 100 watt solar panel on the roof.
I never have to think about battery power. If you boondock, solar will change your experience.