They usually come in the larger Thetford size (our Camping World carries termination caps, straight and angle sewer connections) and the standard size made by Camco, Valterra, etc. I use a Thetford termination cap with the wire to prevent it's loss then use a Valterra adapter left connected to my sewer hose. Those should be the only 2 sizes available. When you go into Camping World, hunt up the white Thetford fittings and compare them to the Valterra fittings. You will see that the Thetford is slightly larger. Ask one of the people there for the Valterra adapters that fits the Thetford and reduces to the 3" standard connection. Either that or use all Thetford connections. If so, I would buy an extra fitting as a spare since you can't always find them.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system