Holding Tank chemicals
While I am no "tree hugger," I do strongly suggest that you use one of the bacteria products which are natural digesters, rather than any of the chemical products. Bacteria may cost just a little more, but it is also more environmentally friendly. With the high volume that some dump station septic systems must deal with, it is good to use what is helpful. You will hear many agruements about formaldehyde and other chemicals and I'll not get into that fight. But clearly the natural products will not cause any problems and they work just as well.
I would agree with Poppa on the gray tank. In warmer weather it would be wise to add some of the product to the gray tank and I would give it at least a 1/2 dose. Gray water can smell worse than the black at times due to the kitchen grease and such in it. But there is no need for a special product just for it.
While you are on the subject, be sure that you use a septic system friendly TP as well and always use plenty of water in your black tank. The #1 cause of problems in the black tank is too little water.