I have never seen a set up like this, my guess on the first valve on the black water is do to the long-ish pipe run. It probably prevents solids from building up and plugging the pipe run. As for the second valve, on the same line, it is part of a standard set up and probably easer to install an extra than create something new.
Any chance you bought this camper used and the former owner installed the extra valve?
2011 Wildwood T26BHXL
2004 Yukon XL Denali
43yrs of camping; one lucky guy