You need to differentiate between what the law in theory says and how it is interpreted and how they will simply not care and knowingly go against it. These are very different things.
If you claim a
nomadic lifestyle and you happen to have a tax agent assigned to you that is a bit more on the a*****e side what will happen is simply that he will not accept your arguments, counter with some bulls**t suspicion based not really on facts. The result will be you will get a tax invoice + back taxes + penalty + future tax payment for the contested time. Depending on time and size also criminal charges.
You will have about a month to pay this. You can contest it but it again they need to approve this and they will not. So you can stretch that out for another few weeks or month maybe but then it will be collections going after you. Depending on where you claim to live / be at that time / your postal address this will result in a variety of things. Will eventually go to european arrest warrant, assets freeze, passport collection should you enter
Germany if you do not pay that amount (which of course you can contest later in front of the finance court).
You absolutely DO NOT want to end up at this point but the chance of this happending if you dont have your ducks in order (yes a tax
certificate and proper country of residence goes a -long- way in this) is very high.
Even if you contest it all and maybe win in front of some higher court later this will take forever and if you did not manage to make them place a hold on the payment you have already lost and its likely to f**k you over for good depending on timeframes, penalties etc we are talking about.
Remember tax law in these countries (Germany) is not based on innocent until proven guilty. It is the other way round and they dont give a s**t -at all-. They will without blinking destroy your life knowing exactly that even though they are wrong - they have the burocratic process on their side. They will with full knowledge use that fact against you.