Just did some work on a friends 06 HR, slide issue, didn't notice the location of the dump valve, but , the belly panels are held in place by about 20 self drilling screws each and sealed along the outside edge with grey tape sealant. 5/16" socket on a power driver and the panel was off in 3>4 mins.
Except for the rear panel that was above the stabilizers, all appeared the same.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.