BigB, I'm towing a 2014 Livin-Lite CampLite 16DBS. It's 20' end to end about 3500lbs which the 2.7 tows the hell out of it. A casita or Scamp would tow very nicely.
I've even used 87 when towing about 3,800 pounds (2,400 pound car, 1,400 pound open trailer). I used 91 for a few tanks and felt no difference in performance, and sow no improvement in gas mileage. I would go 91 if I tow near its max, on hot days, or climbing mountains.