A single rear-axle 40' DP is probably close to its legal axle rating. This number (20,000 pounds) actually is enforceable for RVs, so before considering a rack, make sure you have enough axle capacity. The weight won't be just the rack+quad weight either, it will take some of the weight off the front axle, and add it to the rear. How much depends on your wheelbase and rear overhang.
Have you seen the Idaho Tote? Unlike a trailer, it doesn't need to be backed up. The caster angle of the axle changes when you put it in reverse, so it simply turns with the coach.
http://idahotote.com