We spent the last week of July and all of last August at FW. The first two weeks of August was busy but there were sites available. The last two weeks in August was like a ghost town. For much of that time, our loop (1100) had no more than 3 sites occupied. The only loop that was quite full was the 100 loop, but I'm guessing that was because the 200 loop was closed. We checked every other loop and only one (loop 300) had more than 50% of the sites occupied.
I think one reason for the low occupancy was the free dining. You could stay at a value resort with free dining included for around the same price we were paying for a premium site.
We didn't notice a huge difference during the month with park attendance. Again, I think this had to do with the free dining. Also, if free dining is offered during your time there, be sure to book your dining ASAP as it was very difficult to even find a restaurant with walk up availability at lunch the couple of times we tried.
As for the rain, we have been there many times during August and normally you can set your watch by the afternoon storms. This past summer was really weird... very rarely did it rain in the late afternoon. Numerous times it was overnight and at one point, I think we went 7 days or so without any daytime rain.