I'm sorry, but someone has done you a disservice by selling you a trailer with too much hitch weight for your tow vehicle. IMO your minivan would need a custom hitch receiver fabricated with more and stronger attachment points, suspension modifications, and possibly a sway bar or different tires
. The Casita
may have a 365 lb dry hitch, but once loaded with LP, battery
, water, and camping gear it will be more. It could be anywhere from 400 lbs to 550 lbs, give or take some. If you add a weight distribution hitch, that unit could weigh another 50-75 lbs, which means the hitch then must support that much more. The minivan just isn't made for that much weight on the back end. It needs stiffer suspension in the rear and a receiver that won't tear off under load.
If you were up north, I'd suggest going to Can-Am RV in Toronto. But I don't know who in Florida could do the work.
By the way, I've had two class III Curt hitches (rated for 5000/500 lbs) tear apart on me; after holding up for a couple years of constant towing (but never overloaded) the metal fatigued and gave out. I won't buy another.
If you don't want to replace your minivan, my advice would be to change the order to a 16'. They are much lighter on the hitch.