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.