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.  

A lot may depend on what the weather is like when the time comes to head south. If the temperatures along your route stay above 32F for most of the time it would not matter.

In the last few years weather has been unpredictable except for a couple days before any certain day.

The weather in MA is too cold to de-winterize.  What if it freezes the nigh before you leave?  Why be in a hurry to de-winterize?  Are you planning on stopping for a few days on the way south? If only a few quick night stops, get a 5 gallon - or a few milk jugs of water and set in the bath tub.  This will be enough to flush the toilet and wash your hands. Then de-winterize at you leisure in the warm weather.