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Depends on where the valve is located. At the tank opening is usually 3”, but gray water tanks may have a reducer for 1 1/2” valve.
You also need to know if they are Valterra or Bristol. Best to put eyes on the valves to be sure.
As a rule of thumb, 3” is black tank and 1 1/2” is gray tank. As I remember Keystone used Bristol.
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2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.