Yes Florida has high enough humidity to require a second A/C unit.
Glad that you are also getting the 50 amp service, so you will not be overloading your power cord.
If you where on a 'marginal day' and had to run the A/C most of the time, it would not have time to cycle the compressor off and let the coil thaw out any frost that might have stayed on the cooling coil. And it would keep getting warmer inside the RV instead of cooling down as expected.
Say you get back to the RV and it is 82 inside, you forgot to turn on the A/C when you left. It would take 'forever' on a 90 degree humid day to cool the RV, but with two units, it should cool off really fast.. .
I would get the 15,000 and 13,500 Btu's units. If you are ever stuck in a campground with only a 30 amp service, the smaller unit probably will use less power, and still have plenty to run the microwave ect.
Also some like to run the A/C in the room they are not in. So if it is ducted and you can run the living room A/C during the night, and only the bedroom A/C when you are watching TV in the living room, it will be really quiet.
Good luck,
Enjoy your new RV!
Fred.