Is it permissible for a man married to two women to bring a housemaid for his second wife, who sacrificed and traveled with him for the sake of da'wah, while the first wife could not go with him, knowing that the second wife is more in need of a housemaid and her circumstances require it?

The majority of jurists hold that a husband is not obligated to provide a servant for his wife, unless the wife is of high status (in beauty, lineage, or religiosity), or the husband is one whose wives are not accustomed to directly performing household chores. In this latter case, the husband's financial ability is a prerequisite for such an obligation. Furthermore, the majority of jurists, contrary to Ibn Taymiyyah, do not obligate a husband to equalize between his wives in non-essential expenditures. If a husband fulfills the obligatory financial support, he is not subsequently required to equalize, and he is free to bestow his wealth upon whomever he wishes. Therefore, your husband is not obligated to provide you with a servant, but there is no impediment if he chooses to do so.