Occasionally, a 16' canoe or a couple of bicycles (handlebars turned 90 degrees, bikes laying flat). I keep an Aluminum, 4' step ladder up there for whatever, like reaching the outside fridge compartment for repair/maintenance while the camper is on the truck. The rails of the rack system are great for anchoring a tarp.
Typically, I enlist the help of a passer-upper when putting items on the roof. For the canoe (about 70-75 pounds), I stand it straight up with the gunwales facing the back of the camper, lean the bow against the rear cross rail and I climb up top while the bottom person steadies the boat. Then, I have my gal pick up the bottom of the boat (stern) while I lift from the top. When you reach the pivot point (or the bottom person runs out of arm length), the bottom person lets go. Easy peasy. The difficult part is avoiding the AC shroud, fridge hood, plumbing vent, skylight, crank vents and the rack's cross bars while walking on top of a moving platform. Make sure your shoelaces are tied, before going up. Our canoe fits nicely around the obstacles.
'13 F250XL SC gas 4x4 8', Camper & Plow packages, StableLoads, LT285/65R-18 Goodyear Wrangler A/T Adventure, 18x9 Ultra Motorsports "Phantom" wheels
'12 Wolf Creek 850 TC Coleman Polar Cub 9.2k A/C, 90 watt solar, dual propane & batteries, Maggie Rack