I built and use such a carrier on my van from time to time, such as when I haul three bikes or bring the 4 whlr + bikes on race work days. To sturdy things up, I added side supports which stabilize the carrier to where it is sturdy enough to hook on the lightweight popup camper. With just two bikes aboard, it is NO problemo, and I would have no fear at 85, but keep in mind, my van is a one ton and the hitch seems a bit sturdier than a normal class 3.
As to the distance from the hitch question... Yes, it makes a difference how far away, from an engineering aspect. True, the little stickee says "blah-blah-blah" and mentions nothing about distance X weight, but it would be opening a whole can of worms for the average guy if they did. I'm sure we can all rest assured that there is a substantial margin of safety built in, probably for guys like us who ride the edge of the envelope.
Anyways, f you'd like a pic of the carrier & a few dimensions + pointers on designing your own, I can scan and send.