The Hearland RV Toyhauler with a rear bathroom probably has 4 tanks. It has a front and rear bathroom, so 2 black tanks for sure, and probably 2 grey tanks as well. It might be possible to plumb all grey water into one tank, but my guess is 4 tanks to drain. It also has a 100 gallon fresh water tank.
I have not seen one in person, but am looking forward to seeing one. It is 42 feet long overall, so a long drain line from the forward to rear black tanks is probably not a option that would stay clean all the time. ..
Fred.