Thanks, britttolbert and Planecrazy, for the encouraging comments!
We've just finished moving, to a different place in Virginia that's about 190 miles away. We moved almost all of our stuff with this wagon, in five loads--three using our 16' open trailer, and two borrowing a friend's 16' x 7' box (enclosed) trailer.
We drove faster than usual on each trip, averaging about 55 MPH, so that we could go up, unload, and drive back in one day. The open trailer averaged about 20 MPG round trip each time--probably ~18 there and ~21-22 back.
The box trailer averaged right at 16 MPG both trips.
However...when you consider that a "normal" gas powered pickup or SUV would have probably averaged about 10 or 11 MPG with that trailer--that's still pretty good.
It was kind of unnerving seeing the average that low, for so many miles. However, once I unhooked the trailer the trip averages immediately went back up to the low 40's MPG, so all is well.
-Funkhoss