After my first and only trip pulling a trailer with a wimpy 2 cylinder motorcycle, my mpg was so horrendous, I contemplated never pulling one again. I was averaging around 20MPG with a 1500 Vulcan.
After reading many posts on this forum that pulling a trailer with a Valk didn't effect mileage very much, I was both encouraged and skeptical. Many reports claim that they only dropped from 0-5mpg.
After obtaining a hitch from a great member on this board, I saddled it up and took a test ride today.
Here is the rig....

HF Tagalong, with tongue extended, Loaded with about 60#'s of cargo. (18# bag of charcoal, two full cases of beer, and some other crap I found in the garage.) Tank filled with 87 octane and Starbrite / Startron Fuel Treatment.
Rode 64.3 miles averaging about 55mph, over gently rolling hills along the Missouri River.

Returned to the same station where I started, filled exactly to where I had it before......1.93 gallons!! I couldn't believe it!! 33.33 MPG!! Woohoo!!
Almost no discernible difference from my normal average! Of course I was a little easier on the throttle, but I'm amazed with the added weight of a trailer how little the MPG was effected. RPM's at 60 mph were right at 3000. Needless to say I'm very happy!!