Any input on Noco's BLSOLAR5 5-watt solar auto battery maintainer?

Hey gang. I've got an Eurovan camper, and for the auxiliary battery, I use a 50Ah Optima 34M AGM. We park the van over the winter, and I removed the aux battery and keep it in a corner of my house so it isn't subject to freezing temps. I understand it's safe to keep this indoors because AGM batteries don't emit gasses. I'm looking for something to keep it charged, and I came across this Noco BLSOLAR5 5-watt solar charger: https://no.co/blsolar5, it's $50 bucks on Crutchfield free shipping.
I figure I can stick the solar panel in a window, connect it to the battery, and forget about it until next summer. My concern is that its specs indicate "1.2 - 20Ah" for Battery Capacity, and my Optima 34M is 50Ah (C20), but then the specs go on to say "Maintains all battery sizes" which kind of confuses me.
Does this sound like it can do the trick? Or is it underpowered, and I'd be better off just getting one of their "genius" AC chargers and charging it once a month?
The BLSOLAR5 also appeals to me because it appears during the summer that I can plug it into the cigarette lighter of the Eurovan, and it will help reduce the aux battery consumption a smidge when we're camping. We usually camp in the desert, so plenty of sun, and we typically drive the van every couple of days, and we only rarely have an issue with the aux battery getting low, such that we don't really need a full blown multi-hundred-watt solar charging solution yet ... so hopefully this guy can serve double-duty to maintain the battery during winter storage, and give us a little extra boost during actual camping trips during the summer.

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