1st Welcome to the wonderful world of Camping and Dealing with your waist.
My wife and I do a lot of state park camping and try to stay in sites that provide only electric. Upon arriving and checking in we head to a dump station, empty our gray and black tanks fully. Then fill up with fresh water and put about 5 gal of water back into our black tank.
My wife and I enjoy long hot showers so we usually go to the bath house for showers. Doing this will allow us to camp 4 or 5 days with out filling our tanks. Longer if we use paper plates..
On our way out of the camp ground we drain our fresh water tank down to about 10 gal (less weight) and completely empty our gray and black tanks, then put that 4 to 5 gal of fresh water back into our black tank.
As we approach our house there is a flying J that has a very nice dump station. We pull in top off the fuel in the truck and they will give us a code to use the dump station. We completely empty gray and black tanks, and flush the black tank out for at least 5 min (if no one is waiting). Once thats done our camper is ready for storage.