My 2010 has a connection next to the water inlet for the black tank sprayer rinse. I use a dedicated hose just for that purpose. I suspect your 2008 would have one as well. With that in mind and from many years of RV'ing I have a camco (I believe it is from them, maybe Thetford) that you connect right at the tank drain connection that does something similar and also allows you to fill the tanks and flush them further which would also be good to have if you ever get a clog in the drain lines, which I have never had. I use the tank flush connection now more than anything else but even then not for every time I drain the tanks.
Welcome to the Jayco Owners Forums, lots of good information here for sure.
2010 GreyHawk 31SS