I have a Thetford 18 gallon tank. This is the only tank I have ever owned so I do not know about the design of others.
It works ok but I think it has a design flaw. I can never fully get the tank to drain. It always seems to have about a gallon left inside.
I would check out the design of all of them before purchasing.
Steve, Christy, Anna and Phoebe (Border Collie)
1994 Classic 30'11" Excella - rear twin
2009 Dodge 2500, 6 Speed Auto, CTD, Quad Cab, Short Bed
Hensley Arrow hitch with adjustable stinger
WBCCI # 3072