I think you'll be surprised how little they'll discharge in a few months of winter storage.
It's just a shame that you've had the experience of your house bank losing storage after only 2 years of use(expensive!).
I'm no expert, but you should be getting better performance out of them and if this is a trend, you're doing something,...wrong.
I've had a few episodes of short lived deep cycle batteries, it's always been the result of overcharging.