Take it apart anyway and usually these valves are ABS plastic. You can buy ABS glue which is ABS plastic dissolved in acetone, You cans try finding the crack and building up from the inside (after cleaning). If you need thicker goop to make the repair you can grind up some ABS and dissolve it in the glue and mash it in.
On my modified Scamp
with a underfloor black tank made from ABS I fitted an ABS fitting onto the side of the tank where I needed it and sealed off the bottom exit.
I also made the 1 1/2" tee that comes off the fitting to the 3" valve for the tie in to the macerating pump I installed. I did the same thing for the gray tank in the rear for the 1 1/2" line to feed the gray tank to the pump. Here is a picture of the added plumbing to the valve to the pump. All of this was done using the ABS glue adding the ground up ABS plastic.
The pump outlet curves through the cabinets to this hose reel to drain the gray and black tanks
Perhaps too much information, but you can fix these things if they are the black ABS fittings usually installed.