Just to report... 

Yesterday (08-03-15), we drove from our home in Rochester, NY to Elkhart, IN (nearby family).  The trip was 490 miles.  We drove more than 400 miles at freeway speeds of 65 (NY & PA) and 70 (OH & IN), using cruise control 80% of the time.

Our '74 SCAMP 13' towed like a dream behind our '04 Mazda 6!  (Although we have towed this SCAMP more than 100,000 miles during the past 40 years, this was our first long trip towing it with our Mazda.)  The Mazda held 5th gear + overdrive 80% of the time; another 10% it kicked out of overdrive but stayed in 5th gear; when we came to grades of 5% of more, it downshifted to 4th gear.  Overall mileage (at 65-70 mph) was 17.5 mpg.  (I suspect we would get 20 mpg or better, driving at 55-60 mph,)  

The SCAMP was lightly loaded, but the Mazda quite heavily loaded.  Unloaded, the Mazda and SCAMP sit level on a straight hitch; with the Mazda loaded, the hitch was ca. 1" lower than level.  Today, I bought a ball with a longer stem, so I can raise it--and the SCAMP--an inch higher when and if needed.