Most people are towing their trailer primarily at highway speeds. Even if you're staying below 60, that is still a lot of wind resistance.I suspect that the weight of the trailer will make a huge difference. The added rolling resistance (in the most general sense, including loss due to tire deformation) and the losses in acceleration (much more power needed to get going) is probably more important than the additional drag (if you're at reasonable speeds).
Also, an 'advantage' of the R1 is that its cross sectional area is quite large, so it hides more of the trailer compared to a MY.
Bottom line is that I would expect less than 50% range reduction if the trailer isn't super heavy (like that 9000 lbs beast that was mentioned) or bulky.
The R1T is very much optimized to minimize wind resistance. The typical camper trailer is very much not. They usually have only a tiny bit of consideration given to reducing wind resistance, and they are also usually taller than a full size pickup truck and designed under the assumption that the full size pickup will give it a bit of a draft.
I think towing a big trailer at city street speeds would be enormously more efficient than highway speeds, since wind resistance really is the dominant factor. It's just that nobody really tows that way (typically speaking, there are exceptions like coastal roads).