Update. Turns out this is (I'm pretty sure now) a Perfecto tank, not AGA. Not like it matters much other than me getting the right part. I called Perfecto, and they were really nice, they said the cause was probably the high lighting. I'm still not sure how this doesn't happen to every reefer especially given the short time period. I'm thinking there might have been multiple factors at work like maybe a weaker than normal area on the trim.
They said the brown (oak) trim tanks are more prone to UV damage, that the black trim tanks have a UV inhibitor, carbon or something, in them. That right there is interesting info that I hadn't heard before. But that the black wasn't "uv-proof", just more resistant. Since mine seems to have cracks in non-stressed points, and I've seen a lot of UV damaged plastic (like if you leave things outside in the sun, or plastic trim in old cars), and it seems a lot like that. Just broken down, sometimes even crumbly but at least brittle.
I asked them about painting it to protect it from UV and they said it might work but the paint might attack the plastic too. Personally I think some of that krylon fusion paint would probably be fine.
The trim is a "large" item to ship so I'm trying to get one from my LFS since I guess it's cheaper for them to ship through a distributor than for me to order direct and make a special shipment.
On another front, I'm looking to see if I can buy a piece of 1/2" thick (or so) glass that I can cut and silicone in as a center brace. If this works out, I won't need the new trim, unless I inspect mine further and it seems weakened on the edge instead of just the center brace attachment area. I found a place online that can probably do it (and temper it, and probably have better edges than me), but the shipping again was very high (and this is a smaller piece), so maybe I can find a local shop to do it, or I can cut it myself if I could find the raw glass to cut it from (having them temper it would be nicer tho)