Call the reservation line and ask the Cast Member to add a note that you will also have a tent (and size of tent if large/known). Not all Premium sites have convenient or large enough dirt/ground space next to the all-paved RV site. Trees and bushes are often in the way to provide some privacy between sites. A Fort Cast Member reviews those reservation notes before the day of arrival (requests for a specific site or location, an additional tent, etc.) and they really do their best to meet the request.
The other thing I will add is that when you arrive at the Fort, walk inside to check in and ask if the site they have pre-assigned for you will fit the tent. The front desk has a huge binder of each campsite with dimensions, obstructions, etc. and can use that to get one that works for you. The earlier in the day you can check in and have that discussion, the more sites they can look at to move you (and others around).
So with enough communication (the message on the reservation) and lead time (early on check in day) these things can work out. Just be patient with any CM trying to help. Finally, if you are given a site that won't work after you drive down and look at it, don't be afraid to go or call back to the Front Desk and see if they can swap you to somewhere else.
Bama Ed