Most folks here will tell you to stay away from silicone caulking as the oils from it can make it very hard to get the new stuff to stick to the old. I would recommend a good quality rv caulk from a rv shop. I am not sure how Scamp
installs their AC but on our Casita
there was a thick foam gasket between the AC and the roof. Have you checked the bolts that hold the AC unit in place, again on mine, I think I had to remove the inside housing to get to them. If they have rattled loose, it may just need a bit of snugging down.