Depending on what type of trailer you have and what type of driving you do, I guess rolling resistance from tires and wheel bearings could be a significant loss.
So, towing a box trailer or camper on the highway, aerodynamic drag is by far the biggest loss.
If you have to do lots of stop and go towing, then weight becomes the biggest issue. When towing you can't fly around turns, and you can't avoid braking for some things that you could when not towing, though you can avoid braking for predictable stops even better with more patience from drivers behind you (or less ability to see them
).
I wonder how Pulse&Glide is affected by towing? On level land, depending on aerodynamic loss, the cycles could get much longer with all that extra ballast, and the pulse would be at maximum BSFC unless you have to downshift (or your automatic does it for you). OTOH, with large aerodynamic losses, P&G would probably become worthless.
I haven't towed my camper since before I got into hypermiling, I just haven't had the money to go anywhere.