Is there an elegant way to get wires out back to hitch?

I ran my trailer light wiring out one of the rubber plugs under the spare tire, which gets it under the floorpan but still above the plastic diffuser panel, then fished it out where the hidden Ecohitch comes out and I had already cut the panel.

 

I don't have a hitch or trailer wiring on my Prius, but on my last car -- a Volvo wagon -- I did. I chose to keep the wiring and plug coiled up and stowed in the storage compartment inside the car to keep it protected from the elements. Then when I needed it, I'd do what you are trying to avoid: run the wiring out the hatch, and shut it in the weatherstripping.
My rationalle was that I only tow a trailer maybe 6 times/year for a couple hours each time. Why leave the connector exposed to the elements 24/365, potentially causing the connections to oxidize and cause a bad connection. Additionally, it's easier to keep it stored in the car, and it looks better -- IMHO.
My current tow vehicle is a Toyota 4Runner. It has the plug permanently mounted on a bracket on the hitch -- professionally installed when I bought the truck used. At the end of last summer, I hooked up to tow a trailer, and had no trailer brakes. After some trouble shooting, I diagnosed the problem to be oxidation on the plug connectors. I cleaned the connections, and was good to go. But if I could, I'd move my plug inside the vehicle -- to me it just makes more sense.

 

Know this doesn't help, but...

Since towing with our Prii is more the exception than the norm, I just chose to lay the harness out the hatch like you do now...and keep it stowed the rest of the time. The hitch is primarily for our the Son's bike rack.

2010 Prius Hitch Receivers and Bike Racks | PriusChat

 

I keep the trailer wiring coiled on top of the 12V battery. When needed, it gets uncoiled & out through the hatch.
In use I had a problem of the wiring not wanting to stay neat where it went through the hatch seal.

The solution I came up with is a plastic "retainer" style clip that mounts in a small ~3/16" hole.
I found the plastic clip in a auto parts display where they had many misc.clips of all styles.

After removing the plastic trim across the back floor of the hatch I drilled a hole so the plastic clip would snap in place. Then I used a ty-wrap to secure the clip to the trailer wiring & made another small "handle" out of another ty-wrap to aid in removing the clip.


In use it works great. It keeps the wiring from falling all over the place when the hatch is opened.

 

A hitch kit install video I watched suggested that, that’s how their wiring harness was designed. Considering it’s a temporary thing, maybe simplest approach. One thing, in rain you might get a little water coming in?

 

Ahh, gotcha.

 

Camera's going on the bumper or somewhere, not on the hatch?

Doesn't seem like it would be very hard to pop out one of the rubber plugs, drill a hole through it and pass your wires through (or drill a hole and make a slit to pass around your wires, if there's already a fat connector on the end), goop it with silicone or Plasti Dip, and pop it back into its hole.

If the camera mounts on the hatch, of course that wiring passes through a rubber tube up by its top edge.

 

If you ever feel remorse about the rubber plug you drilled through, you just buy another one.

Also, if the camera or connector on the end of your cable is small enough to fit through the body hole the rubber plug goes in, you've got a tidy way of stowing it if you ever remove the hitch for some reason. Just pop the modded plug out, pull it and the whole cable inside the car, and put an unmodded plug in the hole.