on our 2000 itasca suncruiser 35U the "salesman switch" merely cut power to the motorhonme lights, appliances, etc. it did not keep the batteries from being charged. the converter/charger in that MH was just a 2-stage model. essentially, that charger just kept pumping voltage into the batteries. the result was premature battery failure. i replaced thst converter/charger with a 3-stage "smart" charger resulting in longer battery life. if i were you i'd look to see if the charger is your problem. until you have it checked or replaced i'd either disconnect from shore power or trip the breakers on the converter/charger. i'd also replace the battery you have referenced.
2016 Itasca Suncruiser 38Q