Your RV's Black & Grey Tanks need to be maintained just like anything else in your RV.  It is recommended that they be professionally cleaned out at least once a year.  This will ensure that the tanks don't have dead weight in them. This dead weight is a mineral build-up called: Struvite. If it isn't removed it could cause your tanks to build up weight that can't be flushed out with just your regular dumping at the dump station.  The weight of this build-up could cause your tank to become separated from your RV as you travel down the road to your next destination.  You will need high pressure to break up and flush out of the tank.