Yes there are a number of RV friendly gas stations in Natchez. There is a WalMart on the west side of the highway in Natchez that's pretty easy to get into and probably has the best gas prices in the area (but that changes daily). Also, there are a couple of stations just across the river in Vidalia. And there is also a WalMart station there that's easy to get into, but the prices are usually 10 cents a gallon more than in Natchez (again, that changes daily). Once you leave Ferriday, there's virtually "nothing" until you get to Jonesville. There are also a couple of gas stations there but they are more expensive. When leaving Jonesville, headed west, you'll go around a big gradual curve. From there, it's about 6 or 7 mles to the 28 cutoff. You can't miss it if you're paying attention... There will be a covered rodeo arena on the right, then about a mile to a very old African Methodist Church building that is painted white, tucked away in the vines and bushes, but very quaint and yet visible. Then about another mile to the gas station/house and the blinking light at Highway 28.
If you are into historical things, Frogmore Plantation is between Ferriday and Jonesville and is a unique place to visit. They have ample parking for big rigs and a pretty nice display of plantation life in the mid 1800's. There are slaves quarters, the plantation store, church, cotton gin, etc open to walk around and view.
Watch on your right side from Ferriday all the way to Jonesville, there's one big (and I do mean BIG) bayou that runs along the north side of 84 for almost 10 miles. As a kid, we used to drag moccasins out of there bigger than my leg. Then it was fun, now, I realize "you can't drain the swamp"
Occasionally, along that stretch of highway, you'll see the carcass of one laying on the shoulder. Some are well over 6' long.