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I hope this diagram helps a little.  A narrative of the process with photos follows.

First some background as to why I have taken on this project.  Tiffin Motorhome installed all of the water input flanges in the same compartment.  The input for City water, along with the fresh and hot water tank drain valves, the outside shower hose and the city/tank fill valve are all in this place.  The fresh water tank fill is on the outside wall just above the compartment.  So right in the middle of the compartment was the input flange for the Black Water rinse.  When you pull up to the dump station, put on your rubber gloves, connect the drain hose to the dump hole in the ground and pull the valve handle, the next thing you have to do with your now dirty  hands is open up your fresh water compartment and slobber in the dump station dirty hose to run water into the Black Water rinse fitting.  This bothers me.  The alternative is to add about 5 steps for changing gloves, getting a clean hose onto the fitting and running it to a  water spigot etc.  Not my idea of fun.

My alternative was to move the Black Water Rinse fitting out of the fresh water compartment altogether so I don't even have to open this door in the vicintiy of the dump station.  

First I traced the hose from the input through the bathroom and into the drain compartment.  There was a check valve thingy mounted up high under the bathroom sink. 

This is up high so there is no backwash between the black water tank and the input flange.  This is like the checkvalve/vent on your home dishwasher.  Don't remove this valve from the system as you will slop black water all over whenever you move the motor home. 

This under sink floor pops out revealing the plumbing below.  This is also where the hot water tank bypass valves are located.

The procedure begins with pulling the input hose up into the bathroom.  Three screws pull the flange from the bulkhead in the fresh water tank.  Then you cut the fitting off the hose  behind the wall and prepare to pull it up into the bathroom.  

The hose runs into the space under the floor out of the fresh water compartment, up over the generator compartment, through the floor and up into the space under the false floor of the cabinet under the bathroom sink.  Note that when you pull the hose into the bathroom, you have crossed the point of no return in the project as threading the hose back to where it was would be impossible.

Next you remove the short front wall of the under sink area next to the hot water tank.  This is stapled to a couple of slats mounted to the front and back walls of the cabinet.  Just pop it loose with a screw driver.  You put it back on with either small screws or staples from a T50 staple gun.  This clears up space so you see where the tank end of the hose goes down into the drain compartment.  

Now carefully look at this place in the floor and where it comes out underneath in the compartment.  Double check and triple check where you think the new hole should go.  When you are sure you will get an inch and a quarter Forstner bit and drill a hole through the floor.  (scarey).  

Now all the hard stuff is done.  Just feed your hose down into this hole into the drain compartment.  Go to Walmart and get a can of expanding foam stuff and spray into the area under the sink where you pulled the hose from and then into the hole around the hose into this drain compartment.  Don't overdo it too much as it drains out the bottom.

Go out to the drain compartment and remove the black plastic fascia plate that hides the dump valve pipes.  Note which way it came out so you put it back in the same way. 

Cut off the excess hose and reconnect the end fitting you cut off before.

Eye ball a location in this fascia plate for the flange to mount.  This needs to be as low as possible but above the dump valve pipes.  (I did it wrong the first time and not perfect the second time so look this over carefully.)

Drill a hole for the flange with the same bit used on the floor.  Screw the flange into the plastic.  I mounted a small piece of 1/8 plywood for stiffness but your choice.  Screw the hose onto the back of the flange with some teflon tape for leak resistance.    Remount the fascia plate inside the compartment. and clean up your mess.