Carefree, A&E, Dometics that I've worked with all have spring sets on each end. They are not very heavy, 3 or 4 pounds each I'd guess.
I've hauled 18 to 21 footers on my roof rack with a tailstand and never experienced any problems with bending using a 5 to 6 foot overhang.
"...the most confident critics are generally those who know the least about the matter criticised."
Ulysses S. Grant