a MSD with a 2.5 g holding tank can easily get filled in 2 days with 2 people. As you move south, you may find that you need to be outside the 3 mile limit to pump out overboard or otherwise get a legal pumpout at a coastal marina or port. You might have a creative solution for collecting fresh water, but dumping your head could be a limiting factor as you move along the coast, especially if you stay in the ICW.
Up to 10 days at a time offshore? If you are planning to stay as close to shore as you say, you will be dealing with a foul current from the Gulf Stream going south and need to figure out what makes sense in putting miles under your keel. Staying inside is longer and involves drawbridges that will slow you down. If you are going outside and intend to hug the coast, you will need to understand that there are not that many good inlets along the US coast in bad weather and plan accordingly.
Running down the ICW does not really interest us in the least so we will be ocean sailing. Its my understanding that the Gulfstream 'proper' is well east of where we would intend to be cruising on our way down, although I also understand there are some rather powerful eddies associated with it to the west.
Here in Canada the dumping limit is 7 miles offshore so our course would be taking us more or less 7 to 10 miles off the eastern seaboard or further east/west as conditions dictate.
Keel up we draw slightly less then 2 feet of water so beaching if need be on the lee ward side of a island,spit etc to avoid a nasty frontal system is doable as well as running up a small river/stream etc. I consider anything when one is further than a 24 hour run from land to be offshore so we will for the most part be inshore. One of our larger passages will simply be crossing the Gulf of Maine.