You do realize the roof of a 13' boler is fiberglass, same as the sides.
So yes, a little silicone around the bolt will work just fine.
I have several penetrations like this (solar panel, roof vent, etc) on the roof of my 13' boler and a little dab of silicone has resulted in no leakes for over 12 years.
HI: JKMBOLER... You can get a weatherseal washer for the outside of the trailer. It's a rubber washer bonded to a S/S one and cost about a buck ea. A reg. nut on the inside tightened together with one on the outside, will do the trick sans sillycone!!!
Alf S. North shore of Lake Erie