This is ultimately about a 1968 RV I'm restoring but hopefully has enough plumbing concepts to qualify the forum. Since I haven't found anything to tell me otherwise, I'm trying to follow residential codes/best practices as much as possible in redoing the plumbing. Due to space constraints and for other reasons that would take a lot of explaining, I need to run all of the "greywater" together into one pipe for a few feet. Unless I'm not understanding something, if this was a residential application, any combination other than the lavatory/shower or lavatory/kitchen sink would require 2'' drain pipe. It would be MUCH easier to use 1-1/2'' for all three combined. So, I'm hoping someone can advise me on whether I have the code requirements right and also what would be best in terms of how it functions. The 2'' version could be done. It would be great to have a valid reason for why I don't have to. Thanks in advance.