You could also use the 5 gallon bucket to reclaim some of your grey water and dump it down the toilet to flush out your black.
I would be hesitant as well to hook up my FW system to my black tank flush, I know there is a very remote possibility of contamination, but it's not worth the risk to me. If I can get a pretty good flush at the site using their FW down the toilet, or the reclaimed grey, I'd do that before I would try to hook up the on board FW system to the waste system.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!