There probably is a way to make this work, but I wonder if your time might be better spent on some other part of your project. If you disconnect the RTC module from your circuit when your RV goes into storage, the coin cell will keep it going for a year, or even a lot longer. Not to mention larger batteries, or even two coin cells in parallel. And even in the worst case, it's not the end of the world if you have to reset the RTC at some point. I just got a DS3231 RTC, and the datasheet says when it's battery powered and inactive except for timekeeping, it draws 1µA. You're looking at months and months.