Dog… How are you going to drain that waste tank?
 
Also keep in mind that you have reduced the capacity of your waste tank with the side entrance of the waste line.
 
You will want to make sure that all that wood is painted, the bare wood will not fly.
 
Also from experence, the waste lines will splash when draining into that hopper, and occasionally you might have a back up or over flow if you dump too much water down at once.  This is the biggest pain in the ass that I have to deal with in my trailer!