I had this same problem once!! After Katrina, I had a camper connected to an out board holding tank that the porta let company would pump out, I left the holding tank valve on my camper, open continuous,This caused the liquid to escape my holding tank to fast, leaving behind a pyramid of solids, had to close valve and fill holding tank on my camper, with water too loosen the solids and dump to the outboard holding tank, so after that episode thats what I did, captured into the holding tank on my camper, and did not dump into the outboard tank, till pump truck was on site