Start pressure washing the boat. Start with 1000 PSI, and if necessary work up in pressure. Max I would go is 2500 PSI, I have used up to 3500 PSI and 4 gallons a minute with a commercial boat yard sprayer with a boat which has bottom paint, where we were stripping bottom paint off. Too close this can damage gel coat.
After the pressure wash, and manual scrape with with a plastic scraper. (West Marine carries yellow flexible ones in the Epoxy resin section--used for spreading resin. Us these as an edge to remove the majority of the barnacles, left after pressure wash .)
You probably will have the small calcified foot plate left. First I would try Mary Kate "on and off" Chances are 90% it will do the job. Be very cautious and wear full neoprene gloves, a protective suit, face mask, googles, hood etc. do not get it on the trailer! If it doesn't do the job, then go with 50% muriatic acid. This is really wicked stuff--and can be very dangerous. If you cannot get 50%, dilution is by putting the acid very slowly into the water--again full protection.
Benjamin Moore makes a product called Concrete Pretreatment and Etch M85-00 which is 50% Hydrochloric Acid, The protectant is supposed to prevent damage to metals, but I would avoid getting it on the trailer.
Most likely if it does not come off with the pressure washer, you are going to have to remove the boat from the trailer, and block it up properly to get to the entire hull.