First of all, congratulations on your new to you Scamp!

I highly recommend you drill the rivets and replace properly with new snap caps and butyl tape. The procedure and snap cap vendors are in the maintenance table of contents under Scamp Rivets and Snap caps. 

http://www.scampowners.com/forums/maintenance-table-contents

Second, never use silicone caulk on molded fiberglass trailers. Makes a horrendous mess and will still leak. Use a proper marine caulk such as 3M 5200.

Trying to topically put a cap over rivet heads won't hold up long term and they'll go missing and leak soon more than likely. Snap Caps, rivets, and a roll of butyl tape are very inexpensive. You can do an entire trailer in an afternoon, and you know it will be done right and won't cause problems for years to come. I've had 3 Scamps I've rebuilt and first thing I do is new rivets and caps. My 19 Scamp I have now is a deluxe so it was mostly screws instead of rivets. We just had a series of huge rain storms and no leaks. That wasn't the case before the new caps, and the exterior is looking great with all the caps replaced like when it came out of the factory.