Hi Derrick
I posted the same question on another forum. I also emailed Thetford asking for their reccomendation to modify the drainage system. They answered with the reply that the flush tank shouild be emptied via the pump into the waste tank, this would leave a small residue that could be captured in the filler cap of the waste tank. Ther fact that IMO the system was a retrograde step was tactfully ignored.
I have therefore created a modification that works perfectly.
Obtain a length of flexible tubing (say 9") of a diameter which will almost, but not quite, fit into the drain plug orifice.Remove the rubber bung fromk this orifice and mark the end of the replacement tube to indicate the depth of entry of the bung. Then, using a container of hot, preferably boiling, water immerse the tubing making it pliable twist the tube into the orifice until the pre marked depth gauge has been reached. The tubing, upon cooling will form a water tight fit. I then found a sutable plug to insert into the tube. This 'Heath Robinson' has proved successful and solved the problem of easy drainage simply and efficiently. Hope this helps