Travel from Massachusetts to Florida in March, should I dewinterize here or there.

for me it would depend on weather the tanks are heated, how cold it is where you live, how far you have to travel untill you won't have freezing temps during the day and so on.  

if you don't have heated tanks then I would be tempted to leave it winterized and when I get to the first stop along the way where I am out of the risk of freezing then find a full service campground and do a good flush of the lines and fill it up.  you can use the toilet while it is winterized, Just have a milk jug of water you can use for flushing if you need to.