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!

I was going to set up an outward pipe on the 3/8 plug. I was going to have it go thru the floor of the trailer and run to a spigot or turn off valve  that sets under the trailer so I can drain it and dispose of it properly. I sent an email this morning to the HD rep that i deal with to see if everything needed to be painted. I have no problems painting the wood under there, Part of the reason nothing is set in stone. I get paranoid with that stuff… I probably wont screw everything down until after I pass the inspection… lol