Black tank

We dump before traveling but often refill about 1/4 with fresh water (with the black tank rinser) and add a cup of dish washing liquid and 1/2 cup borax crystals before towing. As mentioned, this helps clean out the tank and the clear crud off the sensors (though rarely all the way on the bottom sensor). Our black tank is in front of the trailer axles and, since we're close to the rear axle capacity on the truck, we rarely, if ever, tow with the waste tanks full.