Of course, you are talking about July, with hot weather and low water. This is not the best time for fishing, but it's a great time for camping and exploring.
The lower section of Forney is only at 1,700 feet altitude. If it were me, I'd focus my attention several miles in, above 2,500 feet. It will be cooler there. Water levels will largely be determined by how much rain we get over the next few weeks. The lakeside campsite is nice, but it is also popular. There will certainly be several groups there each night around weekend periods. I've fished lower Forney several times this spring and the fishing down low has gotten tough with the warming water and lower levels.
Another consideration is fly vs spin fishing. Forney is a small stream, and the upstream tributaries are very small. I'd try and convince my friends to pick up a fly rod. They have several weeks to learn and could take both rods. It's a long way to walk in and not have the right gear to dabble in a jump-across stream.