There are hitches available that attach to the gooseneck ball in the box and provide a standard fifth wheel attachment. That would be the prefered set up . The adaptors that bolt to the 5th hitch and then go straight down to the ball are not recommended by many 5th makers and insome cases void the frame warrenty.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.