At Antornosee, you can park for free for up to two hours with your campervan, making it a perfect stop for a short visit or a rest.
While I haven’t personally overnighted here, I’ve seen many other campervans parked overnight by the lake. Some people leave their vans here during the day and take the shuttle bus to the 3 Zinnen, which is a great way to avoid the expensive toll road to the 3 Zinnen parking lot.
It could also make sense to park here early if you want to drive up the toll road to the 3 Zinnen early in the morning before it gets busy.
While I’m not entirely sure if overnight stays are officially allowed, Park4Night lists the area as a popular spot for campervans, so it seems many people do stay here. I’d recommend arriving early, though, as spaces fill up quickly, and you might need a bit of luck to find a free spot.
check the spot at Park4Night ... But there are also several other spots in the area that are free, as well as official parking spaces with supply facilities like this one.