Need 4" black/gray water tank gate valves

[DianeBH]

Does anyone know where to get the 4" black/gray water tank gate valves?  The ones on my 1973 Winnebago Indian D22 have started to leak and have a broken handle. I have looked everywhere and can only find the 3" or 1 1/2" ones for newer RVs.  Any suggestions on how to convert to the 3" if I can't find the 4" valves?

[gpw9552]

Valterra SKU 6401 and 6401G