Generally, you remove the tailgate when loading a truck camper.
Obviously, if the camper is so short that you can close the gate behind it, you can leave it on.
Some campers have an overhang that extends below the truck bed (my old Lance had the gray water tank there), so they would interfere with the tailgate. Some will technically fit, but if you leave it down, it will likely collect a lot if rock chips. I guess you could cover it with transport wrap for protection.
But the tailgate is not designed to carry any of the camper weight.
The tailgate itself is also dead weight behind your rear axle, which you usually want to get rid of when you carry a camper.
2014 Ford F350 XLT 6.2 SCLB + 2017 Northstar Arrow