I believe the manual say the maximum weight is 30 lbs., but it doesn't specify cabinet or rail limit. I've always taken that to be the rail maximum limit. I have a 6.5 qt. Dutch cast iron pot, so the weights should be very close to your casserole dish. My 6.5 qt. cast iron Dutch oven weighs 10.75 lbs.; 15.25 lbs. with the lid.
The highest amount of weight I've had on the rails was 18 lbs., and I used double racks. The racks sagged in the middle, but I didn't notice any stress on the rails or sidewall.