Battery on or off

Have been told 2 different things so requesting feedback. Own a 5thwheel. We were initially told that if we are plugged in, we should leave the battery switch ‘off’ ( the switch where the batteries are). If left on, it would wear out the battery ( hooked up so no need for it). We brought it in for warranty work and were told our unit was wired wrong ( they fixed this). When we picked up the battery switch was turned off. Even though the unit was Plugged in, there was not enough juice to get the last slide in ( per the mechanic - he switched the battery on and it worked like a charm). Now we are living in it full time and are wondering if we leave the switch on or off. It is currently on. I just checked the battery sensor and the battery is only at 75% ( usually 100%).
We would do a trial to see if we ‘lose power’ ( if we switch the battery off), but currently in lovely Arizona where it got 119 degrees out yesterday. We have dogs do don’t want to risk things going wrong when at work.
Any feedback would be appreciated. We don’t want to wear out a battery if not necessary!!
Side note: pay the extra money to get temp housing vs staying in a camper here in the summer ??. Lesson well learned!