Found it. The problem is, stuff is really crammed in under my van. And where it's not, stuff is in the way, especially around the rear. There's sub frame in the way partly, and the long generator exhaust pipe that is excessively long in my opinion. It could have been directed out the back instead of the back corner but anyhow, I don't care to mess with changing that.
The only place that might work is behind the rear bumper but I have an EZ hitch system (photo below) so I can easily hitch up the trailer myself without risk of damage to the van. The big metal plate IS removed when not in use but putting a 4" pipe there might get in the way of it... I'd have to measure to see. Plus it would be bad if the van had to be towed from the rear for some reason, I'd have to stop and remove the hose holder.
(not my actual setup, just showing the EZ hitch system I have)
I guess I'm going to stick to the plan of using an old style (crappy) semi vented Rubbermaid Action packer for hose storage. I won't store anything else in the box but sewer hoses and such.. so I won't care about germs in it. It'll be stored on the open air trailer so there won't be any odors in the living space. I may get some end caps for the hoses so at least it controls some of the liquid left inside so the box doesn't hold dirty water.
I do want to get one of those Camco drain hose cleaners:
Camco 39072 RV Dual Flush RV Holding Tank Rinser : Amazon.com : Automotive
It would be useful for keeping the hoses cleaner, especially since I have to move the hose from black to grey. It would allow me to do so without unhooking a nasty hose. I can rinse it good at the dump station with the adapter then swap it over to the grey outlet.
I am going to be full timing in this Class B so yeah, I need to have all the right tools to be able to keep up the everyday maintenance on it. When at RV parks, I will keep the grey water hooked up full time with the drain valve open so I don't even have to keep draining it.
In regards to black, I'd have to go out daily and swap the hose over (or rig up a way to link the two but never have both valves open at once so it doesn't contaminate the grey water tank) so I don't even have to undo the hose when I'm parked for a month or longer someplace unless I want to move the van.
I have a 125cc scooter (80+mpg) for a daily driver which is a major reason for having the utility trailer.
The black water valve is inside at the toilet. It has one of these:
Electra Magic Model 80 RV
Albiet, a very old one... but it appears the design has not had almost no changes since the 70's.