Yep, I was told once that you should never travel with a full or nearly full black tank over the road because of the support straps are not sufficient! Also very minimal water in the fresh tank for the same reason and followed by " Why take the chance?" To use the rest room in an RV, you have to stop. so why not stop at a rest area or truck stop to take care of necessary stops? And why carry the extra weight if you don't have to?
But most of all, what is accomplished by filling the black tank for the purpose of cleaning? It's just not necessary! Go camping with it, fill it, drain it, flush it go home or the next camp site!