I use maintainers on my motorcycles (garage) and another vehicle I don't drive in winter (outside but with power available) batteries all have lasted + 6 years. On my sailboat which is outside trailer stored I use a 4-5 watt solar panel, (hanging veritically so not really ideal for sun but keeps snow buildup off it) directly hooked up to the combo start/deep cycle AGM, and it has lasted over 5 winters with no sign of degradation.
Of interest, I've measured some minimal output on that solar panel with over 4 inches of snow on it when sitting horizontal, which was amazing to me. That said to keep it clear, I hang it vertically now, and don't even check it all winter. This is in Calgary with at least down -30F sometimes, occasionally lower.
Calgary, AB Canada