They CAN be put in any loop but the Premium sites (biggest, most expensive) are all concrete and people have to go off to the side which is a little awkward or pitch on the pavement (works best for self-supporting tents).
1500 is my rec for tent loops (If you have a Tent type site reserved) with the other being 2000. 1500 is smaller (21 sites) versus 2000 (60-some) sites and also 1500 is closer to the core of the Fort (pool, trading post, camp fire area) so it gets filled up quicker than 2000.
Bama Ed