Fun fact, Iceland is far enough north that the sun is still casting it’s rays during mid spring through mid fall all day long. Most tourists tend to follow regular daylight. For us, we chose late starts and late nights for many reasons. This has the benefit of avoiding most other tourists, but drawback that everything is closed.
Seljalandsfoss is one of the few falls in Iceland you can walk behind, but keep in mind it’s very wet. Due to the late night visit and lack of tourists, we managed to climb the impasse behind it and do a full loop under the falls. I do not recommend doing this during the day, or you risk less able tourists following you and injuring themselves.
The signage for camping on this road says 1km. The entrance to the Seljalandsfoss parking area is roughly 700m, whereas the entrance the campground is about 1.2km.
Basically, if you see Seljalandsfoss directly in front of you, you’re in the parking lot.
Camp ground was ok with pleasant guests. Bathrooms, washers, and a communal kitchen, but the staff was all but absent. We briefly saw them while making a pad Thai MRE breakfast with egg (below) but by the time we entered the structure, they were gone.