I have seen this used a couple of times when camping where the guy had a regular plastic tote tank on wheels stowed in his truck bed. He had a 3/4 hose adapted to the fill hose of the portable tank and used the FLOJET to empty his RV tank into this while it was still in the truck bed.
Then he said after it emptied then he would disconnect the 3/4 hose from the FLOJET side and bungy cord it high on the truck bed and off to the dump he goes...
At the dump he just uses the 3/4 hose into the dump and gravity feed the contents... No mess whats so ever...
He made a point in saying be sure the portable tote has a capacity larger than the tank you are transferring...