How much weight on gooseneck/5th wheel/truck bed?

OP, where in S Fla are you? The problem I see with that type of trailer is the axles are close to the back so half of the added weight goes on the tongue. If you can find a trailer with the axles further forward so more of the horse weight goes on the trailer axles you might have enough truck. You do not need 25% of the weight on the tongue but by doing this the manufactures can put smaller/cheaper axles and tires under the trailer. I'd look at horse trailers at Stomski Horse Trailers in Lake Worth and help in deciding what will work safely with the truck you already have.
*They currently have this used unit that looks promising as the axles are much further forward than most. Tell them Dick B sent you.