I did a fair amount of research online regarding walk-in tent camping at Heron Lake, during the week. See the great maps and detail at https://newmexicostateparks.reserveamerica.com/camping/heron-lake/r/campgroundDetails.do?contractCode=NM&parkId=430012 . I called the visitor center to confirm that being a walk-in would not be a problem during the week. Once one arrives as a walk-in, look for the campsites with a green tag on them.
The Heron Lake camp sites shown at the above reserveamerica website all have a fair amount of trees for shade. What the reserveamerica site does not show is a few more Heron Lake campgrounds southwest along Highway 95 (beyond the dam). The southwesterly sites are way more exposed to the sun than the first four campgrounds. Otherwise, the southwesterly campgrounds have fine views of the lake.
I was at Island View campground, in a loop that was convoluted enough that I do not think a trailer (with its inevitable noisy generator) could get in. I guess that this is the point. As a result, all I heard from time to time were muffled voices and no generators. It's the first tent spring-summer-fall camping I have enjoyed in years that was generator-free. I had at least one empty campsite on either side of me as an additional buffer to noise.
I walked along the lakeshore. It did not seem that low. I saw large fish jumping in one cove. I hiked the Salmon Run Trail to the dam. The Salmon Run Trail is a beautiful hike, not all level but not too steep for this older person's knees and walking poles. On the lake side of the dam, walking along the highway to get a good look at the dam, I saw several schools of large fish.
The vault bathrooms and the flush-toilet bathrooms were fine. Not super clean and not super dirty. I did not try the shower but it looked like people were using it.
This time of year, bring some bug repellent or wear jeans and a light long sleeve shirt at night. New Mexico has had a lot of moisture this season, so I think the bug-giness is higher than usual. Still, I sat outside and read for an hour or so pretty comfortably.
The camp host was lovely.