Can a person rent the shelters or are they first, come first serve?

Call city of sioux city to make reservations

You can rent a shelter you call the city of Sioux City to do that

You rent them through the city. You don't have to but if someone has then they get it for however long they paid for. I believe you can rent them for whole days and half days.

After digging around it would appear to be first come gets claim to the shelter.

For this particular park, I believe they're first come, first serve. You should call the city to make sure. At least I would anyway.

First come, first served.