>It's gray area legally. If you have income in multiple countries you should pay income tax in each country (how much you earned while working there) but if you are just traveling in a van and don't have any residence and are on the move all the time, not sure how you can do that without it getting too messy. Probably easiest to just declare all income in home base country.

Most countries have agreements to avoid double taxation, meaning you just declare the income in the home country, as long as you spend less then half a year in the other country. The agreements usually have some rules regarding which country counts as the home country if you e.g. own property in both of them.