We have always used tank wand after dumping on past RVs we have owned, although it would be once every fourth or fifth trip out. We did it mostly to keep tank sensors working properly and to prevent residual debris from hardening on side walls, we found dumping and refilling tank a couple of times still left lots of waste behind.
Since it only took a couple of minutes to use wand on those occasions (at home or full hookup sites) maybe 5 gallons or so of water usually does the trick. Last couple of RVs we have owned had built in flush kits on black tank, they do work to a degree but using a tank wand will net the best results if tank really needs a deep rinsing.
