I think I would wash the camper and make sure the soot was completely off before applying the wax.
I honestly can say, I've never had problems with soot on my camper, or my utility trailer from towing with my diesel. Maybe having your camper under roof is not letting the exhaust from the truck blow away, like it would in the open if it were not under roof. The soot does not blow away, so it settles on the first thing it comes to ... your camper. If you can get some air moving inside where you store your camper, you might be able able blow some soot out the door.
I never back my vehicles into the garage. They always go in nose first, so the tail pipe is closest to the garage door, so the exhaust can blow out. If back in, the exhaust would blow against the walls of the garage, making them black over time.
Many, many, many years ago, my wife and I lived in an apartment complex. Parking 90 degree parking spots to the curb and sidewalk. The apartment rules prohibited backing into the parking spots (always nose forward). When I asked the manager why, she said it's because of the exhaust from the vehicles. The exhaust was turning the sidewalks black and also the heat from the exhaust was killing the grass.
When I got my own house, I never backed in for this very reason.
