The amounts paid for travel, meals, and lodging while on home leave are deductible as travel or business expenses subject to the rules and limits discussed earlier. You must be able to verify these amounts in order to claim them. Amounts paid on behalf of your family while on home leave are personal living expenses and are not deductible.
IRS: U.S. Foreign Service Employees
The AFSA Tax Guide is another good resource for this information with tips about keeping a travel log.