Your address is the place to which letters should be addressed so as to reach you.

It may be the same as the place you live in, but not necessarily. And for someone leading a nomadic life, by definition any usable address has to be a non-residential address.

I'm sure that if someone without a fixed address, when filling in an official form gave as their address the place where they happened to be on the particular day, but very likely wouldn't be the next day, that would be seen as breaking the law.