How long does it take to get to 25 gallons or the point where it starts to stink?
You did mention this was your first seasonal site. If you are leaving the black tank set between visits without dumping and it takes a month or more to get to the 25 gallon mark, I'm not sure anything you put in there will stop the smell.
Portable holding tanks are designed to be used and emptied on a pretty regular basis whether full or not. Holding tank chemicals are not designed to work for months. If you leave the stuff in there for any length of time, it's going to smell.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 42