It all depends upon the size of your tanks. I have 3 holding tanks (Black, gray, and galley). All 3 of them are 30 gallon. My fresh water tank is 47 gallons. When camping, I dump my Black and Grey tank every 2 days. That translates to 4 to 6 Navy showers, which is just perfect to drain our black tank and use the shower (grey tank) to back flush into the black tank and rinse it out and drain both completely without over spilling my 32 gallon blue tote. This way, I make 1 trip to the dump station every 2 days, the back flushing into the black tank keeps it clean. (There are 2 of us, no kids).
Here again, it all depends upon your usage, the number of people, and how thrifty you are with water.
Before heading out with your camper, you need to experiment with a driveway camp, if possible, track things like water usage, how much goes in the tanks, and then you'll have the advantage of knowing your camper's limitations when you head out for real. And, you'll learn how to be thrifty with your water usage, get use to Navy showers, and start focusing on keeping mental track of your real usage.