For the gray tank, instead of Calgon we use Borax and Dawn Ultimate dish liquid. 1 c. dish liquid and 1 c. Borax dissolved in 2 c. hot water. We couldn't find the Calgon all the time. Borax is sold at most Walmarts. For the black tank we use Camco TST Blue.
there are 2 of us. We have a 60 gal grey tank and 72 gal black tank. We can go 5 days without dumping if we are careful with showers or shower in the campground bath house. With just one of you and a tank about half the size of ours, you should be able to go 5 days as well.