Avoid actual silicone; it seals fine, but is very difficult to replace/touch up when needed as essentially nothing, not even fresh silicone, will stick to cured silicone. One has to completely remove the silicone and thoroughly clean the area before applying new sealant.
For lights, Geocel pro-flex RV is probably the most recommended sealant. I've used OSI Quad, which appears to me to be very similar, and is less expensive and more readily available at big box home improvement stores.
For roof seams (that are horizontal), the two standard products are Dicor self-leveling lap joint compound (which, when it starts to get cracked and worn, can generally be renewed by applying a thin layer on top of the old sealant after a good scrubbing) and Eternabond tape.
The typical places to have leaks into the cabover area are the clearance lights, the seams at the roof and edges of the cabover, and windowsโparticularly the front window, if there is one.
