I think the part that Brett showed is the correct part. The end with ears fits the larger Thetford fitting on your trailer. The opposite end has 4 studs that fit the ears of your sewer solution. I know that Camping World has the proper part because I purchased one from them several weeks ago. They also have the Valterra part that fits the Thetford fitting then reduces so that you can put a 3" hose on it. It is hard to see the studs in the picture but if you look close enough you will see them. I had to buy this adapter so that I could add a 45 degree clear elbow for mounting of a Flush King I also purchased several weeks ago from CW.
'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