Yours is somewhat different than my antenna was, and I've never seen an antenna mounted on the side.
If I were to hazard a guess, I would say that you will find a small hole behind the ceramic fitting, where the TV wire enters the trailer. Behind the "flat metal" fittings, you will probably only find small screwholes.
As to the big honkin' part (photo 3), it looks somewhat similar to the mount for my TV antenna. On mine, there is a crank inside the trailer that moves the "sleeve" on this exterior box (in which the antenna pole sits). By turning the crank inside, the outside antenna turns, thereby improving reception. (Kinda like your parents made you do when you were a kid
) As to what's behind that part, on mine there was a hole (about 1-1/2") thru which the aforementioned mechanics passed.
Is there a crank inside your trailer in the same location as this box?
As to the caulk idea, it would obviously only be a temporary fix, but anything to keep the water out, I guess. Worse comes to worst, they make these big red numbers.......