I suppose it depends on the rear of your rig, mine has the bed at the back and the join mentioned had a gap which I sealed up but when I put the cam on I had pulled the sealing out and never replaced it, I have since been rummaging around behind the tail lights and even after dirt roads it didn't have much dust in there but I had sealed earlier where the cables run into the body as they weren't done well originally so that may be part of the problem of dust incursion.
I am going to reseal the gap with just cheapy mastic which will allow movement where silastic can open up, just stick gun in gap, run and squeeze but will leave a small gap for water to get away if it gets in via a light fitting or something.
If dust is getting in at the back then I would suggest you have another opening to deal with as well.