Hi Jim! Like your boating friend, I, too, like 3m's 5200 sealant and always carry a tube (white caulk, of course) with me.
Also like Boss RTV 310 sealant (which is what Casita
uses in new construction applications). Only drawback ... 310 only comes in caulkgun tubes ... but if you are looking at a lot of windows
However, trust your boating friend. Your trailer is more like a boat than a traditional stick-built trailer!
Like Suz says ... if you are dealing with caulking a roof application, ask your friend for a "high flow" caulk, which will self level and seep down into even a small crack.
Having said all that, don't forget to get a bottle of Capt. Tolley's Creeping Crack Cure ... which will find, penetrate and fix even a hairline crack.