have found that even when the cut off switch is turned to the off position there is some parasitic current draw.
I
There shouldn't be. That is the purpose of the cut off, to cut off all draw on the battery. Otherwise why have it on the system? The norm is that a battery will drop about 3% per month just because it is not a perfect electric storage system. I use a small solar trickle charge system to keep it from getting too far down between uses and since I run the genny every month, I check the battery water also. I recently replace the flooded with AGM batteries to see how they hold up so the water issue is not any longer. Instead of two 6 volt I now have three 12 volt. More amps for dry camping but time will tell how they hold up in our heat. Most batteries in cars/trucks don't last over 18 months and I have found that 2 years has been about the max with RV batteries. I hate it when I get over to hook up and find the batteries are dead and won't raise the landing gear.
That's what I do and it works just fine.I have my fifth wheel next to my house and plugged into a 50 amp outlet. For the winter, is there any issue other than checking the battery fluid level periodically to leaving it plugged in to the house? Doing this in lieu of using a trickle charger. Would rather leave the batteries (two) hooked up rather than disconnecting and having the possibility of not reconnecting the battery cables in the spring in the right order.