The drop with things running varies depending on various things, but watch for "bounce back" to what it was after the load is off. Gradually, the no-load voltage it bounces back to gets lower and lower.
Take your "morning voltage" before solar starts raising the voltage and when nothing is "on" except the small stuff that is always on. So far same as above replies, but in addition you can establish a sort of rate. If you do the same things every day, you can see if you drop say 0.3v a day or whatever.
You start at 12.7 and want to recharge before it gets below 12.1 or whatever you pick. So next day it is 12.4 and next day 12.1. You can see that coming the day before and plan your next day to allow for a recharge. If you will be away that day you must decide whether to do it a day early or wait a day and be below your target.
So what if you go below your target? Depends how low and how often you do that for battery life. A little doesn't hurt, a lot does.
It makes no diff winter or summer, once you establish your daily "rate" for that camping session. In winter you might drop faster than in summer that's all. Depends. Start at 12.7, next day it is 12.3 so you say tomorrow it will be 11.9. Hmmmm don't want that, better recharge today even though it is only 12.3.
