Myself and several friends all have Kargomasters over campers. You can make your own stackers out of galvanized pipe, cover them with foam tubing, and attach the baseplate assembly with four heavy duty hoseclamps. One of my friends has his set up so the stackers can rotate down to flat when not in use, and his system works great for up to 10-12 boats up there.
When you say "gangplank" do you want to walk up on top of the rack, or along the side of your camper at top of bed height? Kargomaster makes (and sells at additional cost, of course) a rail that mounts under the shell and sticks out about 3" the entire length between the rack uprights. It's enough to stand and walk along carefully, and you can rig a mount for one mt. bike on each side of the shell so they don't have to ride so high in the air. If you're talking about having something up on the rack to walk on, one of the other guys has welded woven steel decking across his entire Kargomaster, so he can walk anywhere up there. The only drawback to that seems to be that it is kind of abrasive on the boats, and you always have to tie to the sides of the rack since you can't get down to the cross bars. Can send you a pic of mine if you want. I have the side rails but haven't ever found a need for stackers myself. Hope this helps.
So many rivers, so little time..........