Is this the Sealand toilet with the integral tank? If so, you might also check the seal in the toilet: if there is anything holding that open, the toilet may run. Is the bowl filling as the water runs? If so, then probably the valve rather than the seal.
I did put a ball valve in the fresh water line to the toilet to be able to cut off water flow for those times you need to be water conservative.
WM does have some Sealand/Dometic replacement parts on-line, probably not in the stores, though.
A search on Sealand Dometic RV and marine toilet should get you the part number.