No, you shouldn't have to take off the rotopax mount, there is just enough space between the mounting plate and the small plate that the bolt applies pressure to to get a wrench (ratcheting wrench would probably work best) over the bolt head and another wrench on the lock nut to loosen it and take off the mount with only having to remove the rotopax. I just installed a 4gal rotopax with a single center mount, I didn't understand the purpose of using the long bolt so I just used the single bolt through the top. With the Chamber Pro II wheels the mount is sticking out all the way and the long bolt would just let the mount fall forward and not rest on top of the stud. With this mount I suspect that if you cause damage to the tailgate you will have to cause damage to the body first. All the weight is on the mount, including the weight of the tailgate.
Here is a 40" Nitto on Chamber pro II (140lbs) with a 4gal rotopax that I installed yesterday. The roto pax mount isn't holding much of the weight of the rotopax but it is applying force to the rotopax against the tire to support the weight of the rotopax. Much like the way the sliding part of tire carrier where the wheel attaches isn't holding much of the weight of the tire but allows you to tighten the wheel against the rest of the carrier for support.
View attachment 276127