The receptacle is a Sierra WH10520 unit intended for trolling motor applications. I obtained this, and the matching plug, at West Marine. The receptacle is rated to handle the peak 20A that the macerator pump can draw. It has 8 AWG leads, and I purchased 22ft of similar cable from West Marine to route to the battery. This gauge wire gives a 5% drop over 40ft at 20A and a 3% drop over 40ft at 10A. This seemed adequate for the application.
I found a heavy duty angle bracket in the deck-building section of Home Depot and drilled a hole through it to mount the receptacle. I covered the cable with a plastic split loom and attached it along the length of the trailer with wire retention clips, using the existing screws with which the thermal cover is fastened to the trailer frame.
The final exterior attachment point is shown below. This is just rear of the front-left stabilizer jack. Here the thermal cover is notched to avoid a cross beam. I routed the cable up into the space between the thermal cover and the Tyvek insulation barrier.
The cable enters the front storage area through the floor near the back wall -- shown at the bottom right of the image below. I drilled a hole through the floor plywood and used a sharpened dowel to punch down through the insulation and the Tyvek barrier, meeting up with the cable exit point shown above.
From my previous solar panel project, I had already created a utility panel to mount the solar charger and inverter. I added another 25A resetable breaker from West Marine through which I connected the external receptacle feed to the main battery lines.