The theory is sound for low iron content fresh water use. Since you are not likely to find that in your black and gray, forget about it. Fresh is more likely to be accurate as long as you are using softened, filtered water, also unlikely.
If you must... try non-formaldhyde additives for the black and gray. Ice cubes down the commode just before towing. Liberal flush king usage for black and gray. Calgon in the gray(s). Dawn in the gray(s). That should work for a day or two.
After you give up, and you will, try this... leave the gray cracked open. Flush the black 3 or 4 times after dumping so you know its empty and look down the pipe to determine when its full. Repeat till you sell it then start over the week after you get your new one.
2003 Designer Legacy 3590RLTS
2011 Bullet 215RBS - Sold
2010 F250SD PSD SRW CC SB
Timbrens, Rand McNally 7710RVND and
Pullrite SuperGlide 2700.