Mine, too, sags a little, but, I don't worry too much about it. A little water left in the bottom (most of that water is "fairly clean" from the flushing process) sloshes around while driving and helps to scrub the tank and keep soft anything that my flushing might have missed. The black tank is not something that I'm too terribly concerned with having absolutely empty after I dump...the flushing takes care of the waste and any water left is mostly, just that, water.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)