As others have stated, chemicals are not needed. Don't waste your money and time fooling with them. Plenty of water in your tank before dumping is all that's needed. If your tank is not full when you want to dump then run water into it before dumping. Dump the black and then the gray to clean out your hose and you're set to go. It's really not a big deal and dumping can be done in 5 minutes.
I would recommend that you splurge on a heavy duty dump hose so you don't get any surprise explosions. The cheap ones are terrible. Also, make it a point to change out your hose every couple years whether it looks good or not. Sun, rocks and cold are hard on hoses.