Lake Ray Roberts Johnson Branch and Isle du Bois campgrounds are run by the Texas State Parks system, not the US Army Corps of Engineers.
I've found nothing but courtesy and professionalism from Texas State Parks employees (in fairness, I should mention that we have served as TSP hosts at another park). Unlike a COE park, you will interact with full-time TSP employees, not just volunteers or contractors.
One thing that many folks, especially out of state folks, object to is the daily Entrance Fee. That $7 per day per person raises the cost of a W/E campsite to $29 per day, not the advertised $15 per day. A person can purchase a Texas State Parks pass for $70 which is good for the daily Entrance Fee for everyone in a vehicle. This pays for itself for a couple if their stay is 5 days or longer.
We like Jonhson Branch better of the two, but nothing wrong with both. Always go to dinner at Babe's Chicken in Sanger.
Reservation - Texas State parks does not reserve specific sites except for a few parks like Lake Livingston. You reserve at type of site, and work with the staff upon arrival to select your site.
Mid-May is an iffy time reservation wise. Weekends can require reservations to ensure the park has space for you. May is typically the end of the comfortable camping season in north Texas. It is getting too hot many years for good camping by Memorial Day.
It can also be a very rainy, thunderstorm filled month in the D/FW area. Watch the weather forecast, and listen to the radio for weather information. Ilse du Bois is in Denton County, Johnson Branch is in Cooke County.
The COE does have a very popular campground just south of Denton - Hickory Creek - which has good access to the north Dallas suburbs. Reservations are essential.
For a north central metroplex area - the former COE campground - totally rebuild after the 2015 & 2016 floods - The Vinyards in Grapevine is a good choice. Though leased/ run by the City of Grapevine - its rates are up there with commercial parks. Reservations are essential.
Southwest of Fort Worth is Holiday Park - a COE campground on Lake Benbrook off Hwy 377 - Reservations are necessary.
Northeast of Dallas is Lake Lavon with three COE campgrounds open that time of year - including sewer hookups at Lavonia and Clear Lake. But those are 30 amps sites. You might need two A/Cs in mid-late May. East Fork has 50 amp, but no sewer connections.
Other reasonable price options include the City of Lewisville park on Lake Lewisville, and Cedar Hill State Park on the southwest of Dallas.
The location of who you are visiting is critical in the CG decision.
My son lives in Decatur, northwest of Fort Worth. My daughter lives in Plano.
The only CG kind of close to both is Hickory Creek, but is it too far for daily visits to either. We easily run 150-200 miles a day on the truck traveling from the CG to either's home and a few errands.
We camp near Lake Bridgeport for several days to visit my son, then tow 100 miles to Lake Lavon to visit my daughter and our doctors.
