I WAS going to use a single spring WDH to tow Casita w/ my Jeep Cherokee, but because I want to leave room on the "A" Frame for my Honda 2000i generator, I've decided to go w/ a 2-spring WDH.
They are offered in either round bar (bars have a 90 deg bend) or trunion mount w/ flat bars.
Other than ground clearance issues, I can see no clear advantage of one over the other. I asked my local RV dealer and he didn't have a clue, either.
Anyone here have any reasons for a preference of one over the other?