Sewer smell in trailer and under kitchen sink

Thanks all for your responses. We are going to check out the air admittance valve and see if that's what may be causing the problem. We always put plenty of water in with the drop in packets and also do a thorough cleaning of the tank when we get home with a long wand attached to the hose. This gets alot of the toilet paper out, which holds smell too. Also may try other things to control the odor rather than the drop in treatments. Have read in other blogs on this site about using laundry detergent along with calgon as well as lots of recommendations on the Happy Camper brand. Again, thanks for your responses and happy camping everyone!