We did not stay at the Dunes but there were signs up when we went there that all campgrounds were full. Don’t know how else to get ahold of them.

We stayed there in late September of 2021 and just showed up. If there is no room, you can always camp at San Luis State Wildlife Area, Mosca, CO. I also remember that it got pretty chilly at night and it snowed up in the mountains while we were there.

When we were they they said they usually try to reserve one spot for someone who just shows up. But all their other rv spots and most of the their cabins were booked

There are a very small number of prime spots at the bottom that likely need reservations. There are lots of smallish irregular spots up the hill that you could probably snag. They are mainly camping as I vaguely recollect. I found a 25’ spot some time ago.