Here's a quick and easy way to possibly find the problem. Since you say they are discharging overnight I assume nothing should be turned on but it's possible something unknown is operating. My first impression is the tank heater. Since you're in Canada it's logical to assume it's cold enough to activate the heater.
Check that first. If it's off then put a voltmeter on the chassis battery. Then, remove and replace each 12 volt fuse, one at a time. If you see a voltage change when you remove a fuse then it's highly likely you have found the problem.
2018 Forester 3051S (aka Clyde)
2015 Wrangler Toad (aka Bonnie)
At some point during every day you suddenly realize nothing else productive is going to happen the rest of that day. For me, it usually occurs around 9 am.