Since you are located in San Diego, you have little to no chance of encountering freezing temperatures, so leaving the batteries in the trailer during storage is not an issue. The question really comes down to the battery manufacturers’s storage recommendations, and maybe what your definition of long term is. Keep in mind that the small handful of people who full-time in Escapes would typically be plugged into shore power most of the time, and therefore would not have the battery switch off, and those with adequate solar power would definitely not have the battery switch off. I started off with flooded lead acid batteries, but switched to AGMs because I was getting false alarms from the propane detector, installed (in my opinion) too close in proximity to an almost impossible to seal and poorly ventilated inside battery box that when heavy charging occured, would apparently vent small amounts of hydrogen. I put a switch on the propane alarm and when at home (in storage) kept the trailer on shore power with the battery switch on, with no obvious detrimental effects to the batteries. Had I been in a cold climate, I would have pulled the batteries and stored them in a warm location on a trickle charger. When I switched to lithium batteries, I had to change my practices. Many lithium battery manufacturers recommend storing them around 50% charge. I purchased Battleborn lithium batteries and their recommendation is to fully charge them (100%) and then cut power. Due to some phantom current draw, I ended up putting a battery disconnect on the positive post of my battery bank (two batteries wired parallel) and I keep the trailer on shore power with the battery switch on. This allows me to turn the A/C and the lights, etc. on if I want to work in the trailer, yet allows me to comply with the battery manufacturer’s storage recommendation.
Having lived half of my life in New England and the other half in the Sun Belt, my experience is that batteries die sooner in hot climates than in cooler climates. But like living things, they all die sooner or later, and in storage they all lose voltage over time even if not in use, because battery replacement is a fact of life in terms of an RV. I personally would not worry if the battery switch is on or off and the trailer is connected to shore power if the batteries in question are lead acid. There are those here who will take umbrage with my position and that is OK because everyone has an opinion, and most everyone thinks their opinion is the correct one that everyone else should embrace as “gospel truth.”
