Just a few general thoughts, first off Grand Isle is the end of the world, 60 miles down LA Hwy 1 south of US90 with few side roads. I think it is a great place to say that you have visited, but I can't imagine spending more than a long weekend there. The area features vacation homes, mixed with fishing boats, and oil industry support operations.
The problem with Grand Isle comes down to it is so isolated, and does not make a good base camp for exploration due to the 120 mile round trip to get to anywhere that is not within sight of Hwy 1.
If you want a Louisiana state park to use as a base to explore cajun culture, then I would suggest you look into Palmetto Island State park south of Abbeville, it is a great little state park, one of the newest in the Louisiana State Park system. The RV sites are separated by dense vegetation, though in the warmer months mosquitoes are an issue.
As to your question about Sam Houston Jones SP, this one is about 40 miles from my house, I have visited it many times over the years, but not since they did some renovation a few years back. The best part about this park is the river if you have a boat as this state park is located at the confluence of multiple small rivers. It also is located within easy driving distance of Lake Charles, and its casinos if any of that is of interest to you. It also makes a fair base camp for exploring SW Louisiana, including the Creole nature trail down to Cameron, LA and back up to I-10.
2002 Safari Trek 2830 on P32 Chassis with 8.1L