Will this be dry camping or do you have 120 volt power?
One problem many have is using up the limited fresh water supply to quickly. That can mean towing to the dump station to dump the grey water and refill with fresh water. Or if you have water hookups but no way to dump on site, a long shower can overfill the grey tank.
Also don't believe that you suddenly filled the black tank to full in a couple of days. That tank gets a tiny piece of TP on the sensors and normally will read full until the tank is cleaned out really well, only to read full again a day later, when in reality it is not even more than 1/2 full. It takes a really long time to fill the black tank.
If you don't have full hookups, and need to conserve space in the grey tank, you can use a dishpan to wash dishes, and either toss that water onto the grass, or put it down the toilet.
Have fun on your first trip.
Fred.