Buy a trickle charger, as long as they are plugged in they should be fine. If not, put it in the basement.
I have a charger that I use inbetween trips to charge the battery, I just take it off after we park the trailer in the storage place and bring it home.
Something like this
https://www.amazon.ca/CTEK-56-158-Au.../dp/B000FQBWCY