Preferred vs. premium is minimal to the untrained eye - basically location (premium being closer to the marina) and size of cement pad (premium being a few feet wider than preferred, although the size of the preferred pad was the norm for decades). For awhile the premiums also had newer grills and picnic tables, although they may have updated the preferreds by now as well.
For the sake of accuracy, there is a bit of incorrect info here.
1. There are only three preferred loops. These are 100, 200 and 300. The 100 and 200 loops are closer to the marina than any of the loops in the Fort. And the 300 loop is closer to the marina than most of the premium loops, too. So, it is not correct that the premium loops are closer to the marina.
2. The concrete pad on the premium loops is more than just a few feet wider than the preferred pads. They are also completely paved and have no sand pad. In most cases the cement pad on a premium site is at least double to size of the concrete pad on a preferred site.
3. The premium sites have square picnic tables and new grills with lids and a shelf. There have been no upgrades to the picnic tables or grills on any of the other sites, and there have been no announcements regarding any intention to make any other upgrades.
Hopefully, these photos will help:
This is a preferred site on the 300 loop:
This is a premium site on the 800 loop (notice all the concrete, and the picnic table and grill):
With a 40' RV, you might want to think about moving up to a premium site.
TCD