the black tanks you shouldn't leave open when camping . they should only be dumped when full . and the grey water should be dumped after the black to flush down black waste still left in pipe. so I see no reason to do what your thinking . jmo
Heres the problem I see with what you are suggesting now that I see what you mean.
unless you are at a seasonal site not all "on site" dumps are the same distance from parking pad so you may be perfect on one and 3 foot short on another .
and you still have to store/haul your rigid tube around .
adding a slinky hose to either end to make up difference just defeats the purpose !