I don’t recommend putting the camper on with the tail gate. The camper should be built to go on a 6 1/2 foot bed without the tail gate. The tail gate is not built to have that much load, and down, the paint is vulnerable to gravel kicked up by the tires. It also is dead weight that would be better substituted for something important, like more beer.
The camper should come with a fold down step that will work great without the tail gate so that would solve the problem you pose anyway.
2003 Eagle Cap 850 Camper
2019 Ram 3500 SRW 6.4 L Gasser